I've come across one or two at gun shows, but I think they were usually $25-$30 each. I finally found one that I was able to dicker down to around $15 (as I recall). It was an after-market mag - looked old but still in new condition in the original package. It didn't have the finger extension like my original does. This was after keeping my eyes open for them for a couple of years at various gun shows, etc. You may luck into one, but if you really want a spare, I'd recommend just paying a few extra dollars and ordering one. I doubt you are going to find one for enough less to be worth searching. Of course, I didn't take my own advice - but I wasn't in any hurry - this was just one of those things I kept my eyes open for.
Doug