I got my shower base from ebay seller
Tri-State Surplus and he's usually got a couple different sizes. I got the 24x32 because it works well with my design and the small size doesn't bother me since I won't be using it that much anyway (campground I stay at has nicer & bigger facilities and would rather burn up their hot water than mine)... Oh, and his are designed for use in RV's and appear to be quality items, I paid $30 for my shower base...
I just checked his site, he doesn't have any shower bases listed at this moment, but check back as he adds new listings as the older ones expire. He does have several RV bath tubs listed right now...