• Seed Savers Exchange - Seed Savers is a nonprofit foundation dedicated to preserving heirloom varieties of American plants, seeds, and edible crops. They are based in Iowa, and you can buy plant cuttings, garlic bulbs, and seeds from them as well as eating beans. They have a large collection of popular heirloom beans available.
• 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)