^^^^
If you are going to have to buy a Snow Leopard somewhere, why not buy a Lion (better house trained?) and also have the important advantage of having Apple Support and, even more importantly, being able to stay in the future upgrade path more easily and safely than an eBay/Amazon route.
Why does it have to be a hard copy? You can copy/move a downloaded installer onto a disc and then you have a hard copy backup.
Lion is not expensive - Under £20? You could even buy and jump straight onto Mountain Lion 10.8.