As far as I know they're readily available at most fish stores. I know they can order them at request. I'm not sure if Petsmart or Petco carry them, as I usually don't buy fish there. They can be found online pretty easy, though. (Highly recommend Aqua Huna.)
As far as size, mine have maxed out at around 2"-2 1/2". I keep them with bleeding hearts which are closer to that 2 1/2"-3" range.