Not exactly what you were after but:
- N64freak has done some fantastic work recently and has 1.4Ghz+128mb ram boards for sale
- With regards to modchip, again, Bennydiamond over on Assemblergames has done amazing development of his Xblast modchip and I think still sells them here
Both users are active members of xbox community and it's great seeing them still actively contributing. I've brought both an xbox of N64 freak and a modchip of Bennydiamond and can definitely vouch for the quality of work and the value for money.