There are quite a few good online options for disc shopping. Disc Golf Center has free shipping and a good selection, you can pick the weight and color of your discs. Other good options include Marshall Street, Disc Nation, and eBay as well. The downside to shopping online of course is that you can't handle the goods prior to purchase, but if you limit the discs you order online to ones that mold up fairly consistently or if you are okay with a little variability then you should be fine. Certain molds I won't order online as I'm somewhat particular about the feel, shape, etc. This is something you kinda just have to feel out for yourself. You can always make requests when you order online such as "I'd like the flattest Firebird you have in X weight" or "I don't like my Teebirds too domey, can you find me a moderately domey one?" Happy shopping!
Sorry, when we say "domey" around here (not an actual word per se) we mean that a disc has molded up with a noticeable dome to the top. Some discs come out flat or much closer to flat. Your preference may vary depending on the particular mold.