Here are some sources to try...
http://www.roadsters.com/engines/#Headers

If you have to make your own, here's a company that has been around a long time and has it down pat....
http://www.headersbyed.com/

Or if you want to go HIGH ZOOT, here are the pieces in 304 stainless...
http://www.burnsstainless.com/304tubing/304tubing.html