Seems that the good folks at the RPF (Replica Props Forum) were having a thread for trading paper props. Turns out that the hardest part of doing a paper prop is making the artwork. For your dime store variety "You're Fired" letter from Back to the Future, threads like this are great. You download the artwork, head over to your printer, and boom, a prop. The Joker card was a little more complex, so the guy who made the artwork not only put together an entire package for the piece (including zip lock bag and adhesive "evidence" label), but he also printed them out on really killer card stock. Did I mention that it's two sided?? He totally nailed the pattern on the back too.
I don't really know much about the genesis of the artwork for this card, but I think it might deserve further follow up. It's way too clean to have been copied from a screen capture. Maybe he got ahold of a similar style deck of cards?? I don't know.
Anyhow, I think it's a great little piece. Nice part of the movie too.