Here are some options for you on eBay:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2455249789&category=34203

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2455383048&category=34203

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2455492645&category=34203

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2454438968&category=34203

I did not see any stock exhaust manifolds on eBay today, but they do show up occasionally.

Some of the suppliers of old truck parts sell used parts, like American Classic Truck Parts has a stock exhaust manifold for sale for $40:

http://www.americanclassic.com/asp/...;iyear=1950&ytype=to&exyear=1957

Good luck.

1954 GMC 1/2-ton Longbed
1971 Pontiac LeMans Sport