Since your in Canada, try these guys
http://www.nizztex.com
They are actually a shop in Toronto that specializes in 300ZX and J30's (both vehicles have the same engine). Maybe they would know a good place where you can get a used mainfold. Give them a call!
The other place I would try (you can find them in any phone book) is Discount Used Japanese Parts. They are located in Scarborough, Ontario. They are a clearing house for used japanese parts, they often have prices well below what other auto recyclers cost.
As for a source on the internet, other than Ebay (which is hit and miss) try on of the sponsors for this board.