I have purchased plenty of aquarium stock online. Plants, inverts, Cories/fish, food (live). In the states, Petsmart and Petco are killing the hobby. The only way to get uncommon stock is to belong to an aquarium society and trade or live in a huge city where the smaller shops can still thrive. I know many areas on the east coast that have little in the way of private fish shops. The negative with buying online is the freight cost. $20 to $50 is what you should expect to pay. Each of my online fish purchases have been between $80 to $130. I have only had 1 fish arrive dead and 2 fish not survive the first week. ( out of ~30 fish/ 10%). Acceptable rate in my book.
aquabid.com I have not had an issue yet. Aquabid is similar to ebay so it all depends on the seller. Just read the comments others have placed and stick with sellers with lots of comments.
corysrus.com. I would suggest that you avoid them. I have heard many negative things about them and it looks like the negativity has finally hurt them as they have little to no stock compared to past years.
thatpetplace.com; I have purchased from them and they did an excellent job packing. www.wetspottropicalfish
.com/ - This place has a huge list of fish, I have never used them but will in the future. This company sells fish on aquabid. If you are looking for some uncommon Cories, this would be a place to look.