Found this place on EBay. Sells stainless flanges for many applications. 426 flanges listed for $120. Found this with a search for 426 Hemi stainless header flanges, sounds like he may have what you need.

GBE MOPAR 426 HEMI 304SS EXHAUST HEADER FLANGES - LASER CUT
Seller : gbemandrelbending

GBEmandrel.com