There are just tons of livebearers that you will seldom see in the LFS.
A problem that many of them share is that they are not as colorful as the more common livebearers that you do see. As has been indicated, I keep lots of less common livebearers including a species once though to be extinct in the wild, Ameca splendens, and lots of the other goodeids, also known as splitfins. I even used my own goodeids in the livebearer sexing thread to show a splitfin and how to sex them. If I recall correctly I used my greens, Zenoophorus captivus as the example fish. Unlike common livebearers, which are mostly poeciliids, they have an andropodium instead of a gonopodium in the male. The goodeids also provide nutrients to developing fry, in a fashion similar to a mammal, unlike a poeciliid which merely provides a protected place for an egg to develop before the resulting fry is dropped.
One rarer livebearer, a goodeid, that can be recommended based on its nice coloring is Characodon lateralis. Try a Google search and see what you think of them. UI really love mine and I recently had them breed for me. We will see how that goes. Another subtly colored goodeid is the A towerii, blue tailed goodeid, another fish that recently presented me with lots of fry.