[Lox]

It depends on what you want. The different plants grow in different regions (e.g. salt rice and corkbulb near vivec and suran), gems can be found as treasure in daedric shrines and elsewhere, and a lot of ingredients come from killing the appropriate monster. So if your looking for fire/frost salts look for atronachs, if your looking for rat meat, look for rats. Pearls can be found in those things in the ocean that look like scallops but are called Kollops. The specialty alchemists and apothecaries probably have a better selection than regular merchants/outfitters. Try the one on the north end of Sadrith Mora, I seem to remember he/she had a lot of stuff.

I went through a breif stint of potion brewing after I read Slackerboy's posts, but lost interest quickly. I've been stockpiling ingredients though, in case I ever want to get back in the business. I've filled one crate with low cost ingredients, I've got a chest about half full of gems and other high price stuff, and 3 barrels full of ebony. I couldn't even carry all the stuff to the alchemist if I wanted to sell it.