The auctions vary all the time, just search eBay for RFM22b. You want the 433 MHz version with the small crystal (the S2 version). It shouldn't cost more than around $6 US. If you're in the US there are several sellers here, the last ones I picked up a few months ago were from a place in Texas I think.
Brian is right, they are hard to get off without lifting pads from the main board. I had to superglue down a pad I lifted on a similar module. Fortunately it didn't break the trace, just lifted it off the board.