I bought a set of Doug's for my COPO
Doug's Headers for GM . You should also check Thorley as well
Thorley Headers for GM. Both have been in the business since the 60s. Thorley is the original Yenko supplier, but I had better customer service from Doug's. Both offer ceramic.
At Rob Clary's suggestion, I had them Jet Hot coated instead of ceramic. The sterling finish is pretty to look at (kind of like a bright pewter) and apparently is more durable and has better conductivity than ceramic. They also sell headers (Headman or Hooker, I think).
Jet-Hot Coatings