As far as Nintendo 64 prototypes go, Martin of NES World is the guy I know with the most, if not the only publically known collector. Right now the only unreleased prototype he has I know of is Glover 2.

There's a Twelve Tales prototype that changed hands last year privately for a supposed sum of $10,000 or so. Whoever has it now has never come forward, I could only hope they will some day but you know how it is with some prototype collectors, they like to remain in hiding and not reveal anything about what they own. I'm still amazed when I hear about these type of trades since I can't figure out how they get their connections when you never even see them in public talking about their collection or anything..