Do you want the actual prototype or just a reproduction cart to play the game?  If you just want one to play, you can probably find one cheap with a quick Google search.  Since that's pretty damn obvious, I assume you actually want the proto.  In that case, there's likely only one out there, in the hands of a private collector.  Not sure who it would be, but your best bet is to actually put a price that you're willing to spend out there for him/her to see.  Keep in mind that if it's a one of a kind proto, you're probably looking at a high 3-digit price tag at the bear minimum.

The leaked binaries were not stolen. The near final beta was purposely leaked by a former Argonaut employee so that emulator authors could get it to work on emulators. The earlier alpha binary was leaked by someone who I assume bought a used computer from Argonaut. The leaked binaries were never actually from a prototype cart or anything.