• Native Seeds/SEARCH (Southwestern Endangered Aridlands Resource Clearing House) - This is another nonprofit that is dedication to conservation and documentation of agriculture and wild plants in the American Southwest and Northwest Mexico. They have a diverse store with many different products, including a small but interesting selection of heirloom beans.
• Purcell Mountain Farms - Purcell Mountain sells organic and heirloom beans, rare varieties of lentils and wild rice, and corn posole. They have the largest selection of any of these vendors. It's quite a list of beans!
• Rancho Gordo - Based in Napa, California, where most of their beans are grown. They also sell a limited selection of New World foods like chilies, hot sauce, and popping corn.
The price for a pound of beans is fairly comparable at each of these shops -- about $4-5.
Related: How to Cook Beans
(Images: Seed Savers Exchange. Beans, from left to right: Green Flageolet, Black Calypso, and Dutch Bullet Bean)