I'd love to know where as well!
short of jeffcowe's auctions (Eyal Katz?), ebay has been dry... though cheribly did have SoftPorn and a few other "common" Sierra titles up for auction complete, and a few others have tossed up a few Sierra games since jeffcowe's auctions closed to see if they could demand those high prices.
Thankfully, the guy who offered the Mission Asteroid small folder for the Apple II didn't see those... he had a BIN at $39.95 shipped. Considering that I lucked into it 2 hours after he listed it, I wonder if he realized that he might have sold it a bit too cheaply?