The only 'dwarf' Boa I have personally worked with is the Hog Island Boa. These snakes generally have a great temperament, are good feeders and don't grow as large as the mainland variety.

There is a lot of debate over the genetic purity of these snakes in captivity as they are commonly bred to 'mainland' Boas. All I would say is if you can see the parents and they are both good examples of a Hog Island Boa then that is good enough for me. This is provided that you are looking for a pet and lineage tracing isn't as important.