I've been looking for one and I have not been able to find any bumpers. Only found "bumpers with back" which isn't what I'm looking for. There are a couple of bumper options around the Internet but they seem quite cheap and seem to have a weird amount of plastic.
There are cheap bumpers online ie http://www.monoprice.com/products/product.asp?c_id=112&cp_id=11213&cs_id=1083111&p_id=9772&seq=1&format=2&qs=forums&ref=cj and that seems to be reviewed by a fellow form member saying its nice.