I've seen a number of Radio Shack stores in my area that have them gathering dust. Unfortunately (by my tastes), they're all the blue/purple model pictured above.
Nyko made a
black model at some point. The black one works with the GameBoy Player. The blue one does not.
PlayCubes don't seem to work 100% reliably with all games. But the black one works perfectly with the GBP, so it opens up to a whole world of games. I never use anything but the black PlayCube on my GBP. It's sweet.
You can get a "Cube Joybox", from most Hong Kong import sites, made freshly for you as you order, but last I looked they stopped offering the "base model" Cube Joybox on their website forms, and were only offering the more-expensive one that doubles as a PSO-compatible keyboard adaptor. But I'm sure you could get them to make you a base model one, if you emailed them specifically about it, and they understood your crazy moon language. They're here to serve. Or at least try.
I've heard conflicting things about whether or not the Cube Joybox works with the GameBoy Player. You could probably ask them about that. See what I said about "moon language" above.