I think they're probably harder to come by because they are specific to the 5x0-series machines, of which there were only really two (520 and 540, plus the extremely rare 550). I was given my 520 without an adapter and managed to find one by asking on Freecycle and getting a response from someone with some old 5x0 bits (they also sent me some case screws, the SCSI adapter, and some spare case plastics), but unless you strike lucky they seem to be hard to get unless you're willing to spend more than the entire laptop is worth getting one from some eBay seller.