I know you can buy them at the store, but I make my own.
What kind of beans? Whatever you have left over.
Caramelize some onions, add the leftover beans, a little water and start cooking. As the beans simmer, you chop them with the spatula and break them into smaller pieces. If you don't like the size, add some more water, simmer and repeat.
Cook until the beans are thickened; about to the consistency of a heavy batter and remove them from the heat. As they cool, they'll thicken.
Some people add some Ro*Tel tomatoes, while cooking, or salsa. Chopped jalapenos add some kick.
Me? I like them on a flour tortilla, or as part of a Mexican meal. They're good as a dip and can be frozen for another day.
I've made them from an assortment of beans, but think my favorite is when made from Great Northern or Navy beans.