If you look hard enough, people tend to give them away. They are unwanted hitchhickers on LR (like TBS) and people don't know what to do with them, except kill them because they destroy reef tanks and can break the glass.
We've kept one in a 10g glass with some plain rocks thown in and a whisper filter. He's pretty happy in there. We've given him some brine and unwanted gorilla crabs to feed on.
I'm not so sure the glass tank is the best idea...you may wish to go with acrylic to be on the safe side!
They are awesome to watch, just be careful when handling them. I've seen a nasty pic here on RC of a guy who was hit by one. Not a pretty sight!