Best bet is checking junk yards consistently within a couple hour radius around you. The bigger chain junk yards have searchable inventories online with photos. High tops pop up every few months. Not too hard to cut them from the donor van, trailer them to you house and install them. You'll likely have trouble finding one on the open market because most people buy them and have them installed immediately. If you do find them used, it's likely somebody else got it from the junk yard and is trying to flip it for a profit...