Over at Hidden-Palace forums, a prototype collector is willing to part with his collection in return for a bit of cash. Among the lot are several Mega Man prototypes, including the much desired Rockman X3 "sample" cartridge:
Mega Man 2
Mega Man 3
Mega Man 4
Mega Man 5
Mega Man Battle Network 2 (debug)
Rockman X3 sample
Mega Man 7
Mega Man Soccer (no gameplay)
If you can afford to drop a $100 or more, I'd definitely go for these; it's not everyday someone is graciously willing to part with rare protos. That said, you can check out the complete details and games at this topic.
Thanks to Keiji Dragon for pointing this out.