The swap N' shop on this forum is by far one of the best places to get plants I've found. It has to be the busiest plant-related classifieds I've ever seen, and with the widest selection of plants from the biggest group of people that generally just want to share what they have with others, just for the joy of the hobby.
Besides the hobbyists here, there are other hobbyists out there as well. When you network a bit with other hobbyists and develop relationships, it seems as though there is always a buddy that owes you one and has the plant(s) you're looking for.
Unless you need plants on a very large scale, the above is the way to go, IMO.
Otherwise, I've heard good things about aquariumgardens.com ( priority mail shipping), and I'm familiar with Lowcoaster on AB and it always did appear to me that he had reasonably priced plants with reasonable shipping. I think I ordered a few Alternanthera reineckii from him when I was still relatively new to the hobby.
"Good judgment is the result of experience, experience is the result of bad judgment." --Mark Twain