Off Meta Builds to Try
Cleanse against CC
Barrier (Preferred)
Heal (Optional)
Ignite (Possible)
Exhaust (Against assassin, maybe. Probably don't want it in general)
Berserker Boot
Collector (steal shutdowns when behind and it sounds like you are behind often as Tristana)
Navori
Infinity Edge
Mortal Reminder (If you are new ro the game almost nobody gets anti-heal, so this item really shines)
Guardian Angel (Defensive item, better if you are ahead. If super behind, can trade for an aggressive item. Making easy picks when behind is the way to make comebacks)
Optional Items
(Item ordered by compatibility)
Omnivamp boot is better when ahead to protect your shutdown.
Lethal boot if you want to assassinate (perhaps for shutdown on q squishy)
Defensive boot are also good if they look like they can make a comeback by killing you.
Bloodthirster early but can delay building it. Better when ahead against sustain damage.
Shieldbow in 4~6: Defensive item against burst damage when ahead.
Maw in 4~6: Defensive against AP (generally built when ahead)
Runaan in any slot: Safer positioning to hit cluster of targets in teamfights.
Phantom Dancer 4~6. I consider movement speed utility that I build when ahead in general.
Essence Reaver in 3~6: Burst item against squishies. Great for roaming and picking around the map.
Magnetic Blaster in slots 3~6 (good for extra range against high mobility or long range champions).
Serpent in 4~6: Anti shield burst item.
Botrk in 4~6: Anti health tanks
Let me know if you'd like matchups.