Yeah, you probably won't see these for sale without the coral. Your best bet is to keep an eye out for them at an LFS or see if any fellow reefers are willing to part with theirs. I had a friend who was pretty apprehensive of any crab, so I'm sure I could have convinced him to give his up if he ever had any. If you're in real good with an LFS, you may be able to convince them to transfer a crab from one coral to one you're buying. Like julief20 said, though, they can be very difficult to remove, so most places are unwilling to do that.