I think this website is great. The plants are cheap and, as I read, they're high quality too (read the section "About Us"). Some plants are more expensive but that's understandable taking into account their size and difficulty to grow. But overall I think this website is great. They're located in MA. They even have plant assortments for various tank sizes - they send everything ready for you. I think I might order some anubias, java ferns, and java mosses there - they're all low light and cheap on this website:
http://www.aquariumgarden.com/index.php