WTB m4/m16 Upper Receiver or Upper Receiver Rail

I need a new upper receiver (preferably metal) for my m4. I can't find any sites that sell just the upper receiver and the only ones I can find in Canada are ridiculously overpriced. If you happen to have one or know a place where I can buy one your help would be appreciated.

UPDATE/EDIT

I did some tinkering and I don't need the entire receiver, but more specifically the top rail piece that bolts to the upper receiver. If anyone has the full receiver I may still be interested depending on the price, but I really only need the one part.