As i quickly breezed through my local Goodwill store in East Liverpool, OH i noticed at the checkout line a table with 20ish Shrink Wrapped Munchkin games for $4.99. Not sure if these will show up in any other area goodwills or not. But at that price i figured it was worth a mention.
(Grabbed 3 for trade bait myself)
If you find a way to trade them, let me know. I can't get rid of mine.