The most critical (i.e. hard to find) factor is the 8" distance of the drain hole center (from each edge) - which is 11.3" from corner.
The 36"x36" overall size is pretty common, but fitting the other dimensions (18.1" x 25" x 18.1") exactly may be less common (I believe this is a relatively minor problem).
List of Shower Base (and/or Kits) Source I Have Found So Far:
