The garden centers of the big box home improvement and super discount stores are great places to find small specimens of the more common Mammillaria species. Whether you live in Maine or California, you can always find about 11 or so species in two or four-inch plastic pots, usually sopping wet from daily watering.
The Mammillaria duoformis F. tenango de valle is one such bargain. It was covered in wet soil and debris, and wasn't even in a pot. The reason I wanted it was that although I'd seen M. duoformis before in the garden centers, I'd never seen the tenango de valle form, which has white flowers rather than the red flowers of the M. duoformis regular form. I especially liked this cactus because of the very dark, hooked central spines. It has yet to bloom, but it has already produced several clumps.
The M. duoformis F. tenango de valle is from Puebla, Mexico, and requires light shade in Phoenix and protection from frost. It is a columnar cactus with age, but stays small, getting to about four inches tall.
I think it's worked out to be a good eight-cent investment.