I'm sure there are other good options, but I might recommend Southern Exposure Seed Exchange for pinto beans. No kidney beans, though. I've gotten popcorn from them in bulk before and maybe they would consider that if you ask.
www.southernexposure.com
It looks like Eden Brothers sells beans in bulk. I've never bought from them but it looks like they still have Tongues of Fire which is my favorite bean to grow and eat. They are sold out of most other varieties.
Grow Heirloom Beans - Plant Tongue of Fire Bean SeedsVery popular, high yielding bush-type plants of green pods with red and scarlet colored streaks. Beans themselves also have the distinctive red streaking which makes for a fun ingredient in salads and soups. Fabulous flavor is retained whether...
www.edenbrothers.com
Vermont Bean Seed Company has kidney and pinto beans in smaller quantities but the larger packets are sold out. I have also never ordered from them.
Well-known as a powerhouse of nutrition, beans can also be a foundation for imaginative dishes. Abundant with soluble fiber, they combine well with grains. They are virtually fat-free, have no cholesterol and are a source of vitamins. Their mild flavor makes them perfect partners with spices and...
www.vermontbean.com
And if you want A LOT of seed you could try a place like urban farmer - 5lbs for $30 bulk pinto and kidney.