I have bought plants from Petco and Worlds Under Water. I haven't had problems with either. I don't buy from Petsmart anymore, since all they have now are the ones in the little plastic containers. I have also ordered from aquariumplants.com and from the swap and shop on here. Aquariumplants.com, or other large online retailers, will have the best selection. You can get pretty much everything you want at once. The swap and shop here will usually have much better prices, and some of the rarer plants, but you will probably have to get your plants from several different people. Some of the power sellers do have a decent selection and good prices, so be sure to check out their special sales section.
Post pics of your tank when you get it going. I would post pics of mine, but they look awful. :P