I'd suggest that the quickest answer is get down to the LBS and see if they have a cassette in the scrap (as in having fitted a replacement for someone) that you could have.
When I was running an LBS I had a bin in the corner with all the scrap metal items - chains, cassettes, mechs, chainrings - whatever was metal. Customers were welcome to poke about for bits and pieces (in return for a packet of biscuits
) When the bin got full it went in with the scrap that the local cycle charity got cash for.
Rob