Pupfish are pretty much illegal to maintain in the home aquaria, with the exception of a couple species. Most are, at the minimum, endangered or threatened with extinction within the United States and similarly in Mexico. The main exception to that rule is Cyprinodon variegatus which can be found in coastal regions and estuaries nearly everywhere. Also at least one species, Cyprinodon alvarezi is extinct in the wild so the only populations that remain are those kept by hobbyists.
There is also some discussion and controversy over Aphanius species found in and around the Mediterranean should be categorized in the same class of fish. For the hobby, however, KillieNutz Online separates the two groups.
You will find a list of all Pupfish to the left (as I add the pages)