Might try checking some of the local motorcycle builders.. or tuner crowd?
It will probably cost a pretty penny though. Basically having to cut a pair of stainless flanges (that in itself could cost about $100). Then, cutting/bending/fitting/tacking a bunch of tubes by hand.. making sure they clear the engine mounts, starter, clutch linkage, frame, other accessories..etc. They'd also have to form a stainless collector for the other end. Finally, it would probably get TIG'd together. I could see a set of custom built stainless headers going for over a grand.. easy.
What about taking a current off-the-shelf set of long tube headers and having them Jet Hot Coated.. or something similar?
Another option, might be to use a set of stainless shorty street rod headers? Or.. perhaps a set of stainless long tube headers for another vehicle, that are similar enough.. that they can be cut/modified to work?
Is this a show rig?