Mexico's markets offer the best dried beans in the greatest variety of anywhere in the galaxy. My personal favorite are the Peruanos. They're sold mostly along the Pacific side of Mexico including the interior mainland, and you can get them in practically any size market. In dried form - in bag or bulk - they are the size and shape of small pinto beans, but whereas pintos are pinkish-tan with dark brown specks, Peruanos are a uniform pale greenish-yellow color. When cooked, Peruanos turn a light tan color. Their flavor is as delicate as their hue, milder than pinto beans with a creamier texture.
I'd love to include a photo of Peruanos to document their yummy yumminess. But I ate them all. The beans, not the photos. So go here to see what I'm talking about.
I haven't yet found Peruanos in any US market. I have no idea why not; they'd be popular with lots more than Mexican food. Below the fold you'll find GB's basic recipe for cooking Peruanos in a pressure cooker, though if you can't find any Peruanos you could experiment with another bean variety of your preference.