eBay?
I agree, EBay is a good source of amiga systems/parts.
BUT you have to be very carfull! Rip-off's happen quite often on EBay. Be sure to check the history of the seller, I would not buy from anyone that does not have a good history or no history (first auction).
I purchaced a new mb for my A4000D last year and have had no problems with it.
I paid $250 including shipping for it. I got it from a dealer (can't remember the name).
loki :-D