You can find them on craiglist but most often you'll find people selling tires with rims included. So that makes it hard to search for them and you just have to go through a whole bunch of tire listings until you find something. Doesn't help that you get people listing tires that "fit 17" rims" etc which gums up the search results.

Also can try boneyards though often they think awfully highly of them.

Good luck, they're out there