After spending almost two weeks looking for a place to buy this, I wonder if it's custom made after all. Damn, that *would* be a nice desktop decoration.
It's an epoxy resin, not glass. If you look carefully at the bottom photo, you can see a line going around the whole cube near the bottom. That's where the lower portion of the cube was poured and the fish set on it as it hardened. Then the rest was poured over the bottom.