Once in a while you'll find them on eBay, but they violate their policy (even though "high-capacity" limitations of 18 USC § 921(a)(31(A) &(B) specifically exempted rimfire magazines, and that law sunsetted in 2004 anyway, eBay still doesn't allow you to sell any mag over 10 rounds) so as soon as the listing is discovered they "delist" it and send a nastygram to the seller - if he tries to list it again he gets banned. If you place a bid the auction will need to run its course, and given 7 days they're going to find it, so the only way you'll get one is to pay the "Buy it Now" price if that option is given. Search the "completed" listings for 95712 and you'll see that they often used to manage to get by when descibed as "holds 10 rounds" (at ~$75) but not so much anymore, I think eBay's making a point of looking for them now.
I heard a rumor that a guy ~15mi from me has three for sale at $45 ea, still waiting to hear back. If he still has 'em I'll buy all I can and post back here.