These days, most supermarkets sell fish sauce (look for it in their Asian section). Personally, I have found the best type is usually sold in Asian food stores. You'll find a good selection of fish sauces at nearly any Chinese/Vietnamese/Thai food store. Look for tall bottles with "Fish Sauce" and the ingredients displayed on the label (fish extract, salt, and water - other ingredients aren't necessary). It should be made in Thailand or Vietnam. You can also order it online: Squid Brand Fish Sauce is a good one. Vegetarians and vegans can purchase "Vegetarian Fish Sauce" (made from a mixture of soy bean, chili, salt, and sometimes vegetables and other ingredients). However, I've found you'll get more authentic taste using Thai golden mountain sauce OR a combination of this sauce and soy sauce.
For more information, see my full article: All about Fish Sauce and Vegetarian Fish Sauce