I've unlocked platinum slots on some of my gear but I have no idea where to find chips to put in it
1) Buy from the black market
2) Salvage crater gear which is from tiamat
3) Salvage 3 star costumes
4) Crew dungeon stage 6 (I think)
How are the rates? Do they appear often enough that it's worth farming for them?
Buying from BM is basically dependent on who is selling but over a few days and at most a week you should see all the chips on the BM. Price differs depending on the chip though.
Salvaging crater gear gives latest chips, dual chips and platinum chips. From experience, there is equal chance for each type of chip.
Salvaging 3 star costumes gives new, latest, dual and platinum chips which makes this the worst method to farm chips in my opinion as new chips are a possibility.
Crew dungeon guarantees a platinum chip with equal chance but it is only once a week.
If you are FTP then method 1 for the cheaper chips, method 2 for the more expensive chips ( sell the chips you dont want on the BM),
If you are elite or plat player then method 2 since it forces you to do tiamat which is a gold mine. Once you have enough credits then you can buy the chip you want off the BM directly if you are that unlucky not to get the chip you want.
Farming-wise Tiamat dungeon and Purification Hell/OF can drop platinum chips as well. Extremely low rate though, but you run those for money and gear anyways.