Don,
I checked our downtown and off-strip binders and found cards from Fitzgeralds, Boulder Station, The Orleans, Palms and Rio, as well as the Landmark/Paddlewheel card.

The Paddlewheel card has the stamp box on the back, most of the others have a bar code (I didn't check all of them).

My best guess is that I picked them up either at one of the souvenir stores downtown or at the museum at the Trop...though it's vaguely possible that I got them from a postcard dealer.