Two commonly available species of Viola are
Viola cornuta and
Viola tricolor. They both come in a ton of different varieties. Places like The Home Depot carries these 2 all the time.
My favorite of the 2 is
Viola tricolor.
Occasionally The Home Depot will carry flats of an Australian species of Viola called
Viola hederacea.
Another species that I know that some garden centers such as Armstrong Garden Center carries is
Viola odorata, which smells nice for a small flowered plant.
Of course I mentioned,
Viola labradorica.
There are obviously many other species of Viola, but they're more difficult to find and are rather obscure.
Good luck in your searches, these are cool little plants.