Like vetrx, rooster booster, that type. What do I need on hand and what do I not need? Thanks!
I generally go with clean water, chick starter or other appropriate food, and that's about it.
If I need a towel or something, I grab it from the house (usually the rattiest one available), rather than keeping separate supplies just for the chickens.
If I ever need electolytes for chickens (hasn't happened yet), I will look up a recipe online and make it with things from my kitchen. If I ever have a specific nutritional deficiency in a chicken, I will research and buy the right product at that point. But a "complete feed" for chickens will usually prevent all nutritional deficiencies, so I've never seen any.
For treating specific problems:
Coccidiosis is relatively common. and can kill chickens quickly, especially when they are young. So if you are new to chickens, maybe keep Corid (amprolium) on hand to treat that.
I can't think of any other medication that is commonly needed in a hurry. I would guess that at least half of chicken owners will never even need to use Corid, so if you have easy access to stores that stock it, there is no reason to buy it unless you do need it.
[Edit to add: there were two threads with the same question, so I answered without seeing the other responses. Once I realized there were two, I asked a moderator to combine them into a single thread.]