The thing to remember with with supports with Tristana is that they need to be able to do at least one of two things: Either able to poke fairly well (since Tristana does not), or able to help Tristana's kill combo either by increasing the effectiveness of her auto-attacks or locking down an opponent so they cannot escape. The role of the support is also known as the Carry-Carry, since they are a major deciding factor as to how the ADC turns out late-game.
I will be ranking Supports on a scale of 1-5 in terms of how well they work with Tristana.
- Not that good
I will also be ranking supports in how difficult they are to play in combination with Tristana.
Alistar: 4 - Difficulty: 2
Great with almost any ADC, you have double the positioning between his pound and headbutt + your W into R. Let Alistar do the zoning for you, he's a strong support that can keep the enemy back while you farm freely. Just make sure to always be within jumping range of him, in case he needs backup or sets up a kill for you.
Blitzcrank: 4 - Difficulty: 2
Similar to Alistar, with less of the risk and tankiness. Doesn't have any sustain for you like Alistar though, which means more gold on Health Potions if you find yourself being poked a lot. Blitz's pull, punch, and silence combo is great for setting up your Rocket Jump, and likely will be a free kill unless they flash or other strong disengage ability ready.
Fiddlesticks: 1 - Difficulty: 3
Good for poking and annoying the enemy, and his R + your W means they will take almost full damage from it the ult unless they flash. If you are against a team with more sustain or poke, however, prepare to play passively. Fiddle's only job at that point will be sight wards so you don't get ganked/dove.
Janna: 5 - Difficulty: 3
A good overall support, with strengths that tie well with Tristana. Her shield will amplify the damage you do with your Q going significantly, and it will also protect you somewhat if you dive deep with your W. This of course means that you both will need to coordinate the use of her shield and your Q. She also gives you a passive movement speed bonus, which is nice since you won't be starting with boots. Her ult and your ult can be used in a variety of fun and interesting ways, either for engaging, disengaging, or moving someone way out of place.
Karma: 3 - Difficulty: 3
Shields, Heals, and speeds, but rarely do you see a good one. Her Shield + Slow + AOE dps/heal can make for a great combo with your W+Q+E onto an enemy. If you have good communication with your Karma, both of your combos will be devastating every time they are up, but will not be very effective on any teams that have a hard CC.
Leona: 5 - Difficulty: 2
The Double-Dunk. Very aggressive lane that can be very scary for your opponents to try and trade with. Just hope that she knows when to engage and when not to. You will be the one having to zone/poke if needed. Make sure to activate your Q as soon as you catch an opponent with a stun, but don't use your W until Leona has used all of her lockdowns.
Lulu: 4 - Difficulty: 2
Annoying for anyone to play against, and sometimes to play with. Lulu has great poke and tide-turning abilities for duo fights, but her poking can really f*ck up your cs sometimes. You can get a surprising amount of burst on someone though if you can time all her Pix, Shield, and ult with your W+E+Q combo.
Lux: 4 - Difficulty: 3
If they can land all their skill shots, awesome. If not, NO. Lux is a higher skill cap support than most of the others, because all of her abilities are skill shots. That being said, she can make for a great support if they can land most of them. Her E can act as a temporary ward and be a CC, and has a very long range. Her Q is a root that locks down up to 2 players, and in bot lane generally you will be facing 2 champions. Her W is a shield that can absorb damage on the way out and then refresh if you catch it on the way back. Lux herself is not very tanky though, so you will have to play from a distance most of the time and engage on things yourself.
Nami: 5 - Difficulty: 3
See "Lux" description. Also needs to know when to use their spells on you and when to use them on the enemy, but overall not bad if they can land their skill shot. Her E is great combined with your Q for some very fast and hard burst on an opponent. Her ult is great, easy to land, and works well with your kill combo.
Nidalee: 3 - Difficulty: 2
Poke lane, which you won't be able to effectively do until later on. However, has a heal to sustain you, and her traps can really make your W + Q combo melt enemies fast early on. If she is good with her spears and traps, she can poke someone down and you can finish them off quick. A more difficult support though, since it's very reliant on skills shots and position of not only the champion, but also the traps.
Nunu: 3 - Difficulty: 1
Not as good as one might think to be paired with Tristana. Nunu is one of the few supports currently that brings not a whole lot to the table in terms of what Tristana combos well with. He can give you an additional attack speed/movement buff which will be helpful with no boots early on, and can disable the enemy ADC with his snowball, but he needs to auto-attack minions in order to get his free spell passive snowball, and with your lane already automatically pushes because of your own passive (E), this can be annoying and dangerous. His ult is good if you can get a surprise explosion from a bush (which then makes your kill combo that much easier to land), but more often than not currently supports are warding almost all the bushes in bot lane just so they have vision of what is lying in wait.
Orianna: 3 - Difficulty: 2
Shields and speed boosts make for safer plays early game, and her ult can line up your W rather nicely for a kill combo in duo fights. Outside of that, her ball can be used as a temporary ward and cc similar to Lux's Q. The difference being Orianna has a little more control over it. Her shield will make your Jumps a little more safe, and the speed boost it can turn into is great if you need to disengage fast.
Sona: 5 - Difficulty: 2
Good poke and sustain, but can be very squishy early on. Her aura management will be crucial to the Damage of your Q, and her R + your Jump into kill combo can be really dangerous for your enemies. Has a mediocre disengage aura to speed you away, and since you won't have boots early this can be helpful.
Soraka: 4 - Difficulty: 1
Awesome with almost any champ. A sustain bot. Heal, free mana, and bonus armor/resists. Also has great poke/zone ability. Her ability to give you free mana means that you will be able to use your abilities much more often, and with one of the smallest mana pools and mp5 ratios in the game, that is very convenient for Tristana. Her bonus magic resist helps against opponenets with AP damage, like Ezreal, and a majority of the common supports.
Taric: 5 - Difficulty: 1
Great with almost every ADC. Increases your armor, can heal you, able to stun the enemy and then reduce their armor to shreds, while you blast them away with your Q going. His kill combo paired with your own makes you incredibly dangerous for a short period of time. You will have increased AD, they will have reduced armor, and you will have a lot more attack speed while they are stunned. This is almost a guarantee first blood at level 2, or at the very least a guaranteed draw on summoner's spells from your lane opponents.
Thresh: 4 - Difficulty: 2
Something of a hybrid between Blitzcrank and Nautilus, Thresh becomes very tanky, provides a ridiculous amount of CC, and a provides a Shield/Quick Escape for you. Great option for when you want to dive fast and get out faster. Has no sustain for you however, so more gold spent on health potions for sustainability. If he gets someone gripped, let Thresh CC them for as long as he can before you jump in with your W. If he activates his ult, you can jump ahead and blast them backwards into one of his other walls for more slowed time on them. Likewise, if he uses his ult to disengage and someone slips by, you can knock them backwards and into one of his walls, securing an escape if it were not already secured.
Zyra: 3 - Difficulty: 3
Zyra is similar to Lux in regards to the job as a support for the ADC. She has an advantage over most with 575 range, allowing her to poke much easier on ADC's with less risk, and has a skillshot-based CC that can help save you or secure kills on opponents. The difference between Lux and Zyra however is that Zyra has no way to prevent damage done to you like Lux's shield. She also has no sustain for either herself or you. Essentially if somebody picks Zyra, they need to be absolutely sure of themselves, because it's a kill lane combo. And if you are a kill lane combo that doesn't kill, that's bad news for you.
Special thanks to qazwaxman for the catch on missing this champion, hat's off to you :)