I've found O-rings from the hardware store to be a near perfect match. You just need to match up the size. There's a specific number to look for, but it's been a while since I've had to do this.Can't you, in theory, replace a broken Joe o-ring with a rubber band?
EDIT: That's what I get for typing before reading the rest of the thread. I see this has been addressed already. My O-ring size of choice is 10. Look in the plumbing section.