I highly recommend http://www.plantedaquariumscentral.com/
. I have never had an issue with Heather's plants, and they are always beautiful and ship really well. I just ordered three plants and she gave me a fourth. She usually always adds a little extra of something in there. Highly, highly recommend her.
I recently decided to buy from a different online vendor. While I saved some money, it took me over a month to get my plants, they forgot one of them, and they were all in horrid condition.
I can't speak highly enough of Planted Aquariums Central.