@IntenseCookie Either Home Depot or Lowe's. I'm sure a large hardware store with a garden section (or a nursery) will have one. I think his dish is a 16'' -- the largest size the store had.
@Mort I have a reptirock, it looks nice, but an incredibly awkward size and shape for my tortoise. He lives outside, so dirt gets in his water and I have to clean it daily, which is difficult as mud gets into the corners.