I'm sure various sponsoring venders have these headers but you can also get them from larger mail order places. Below is a copy from one of my a prior posts. Check around see where you can get the best price.
Borla headers for $1,179 summit.com p/n BOR-17113 -> T-304 STAINLESS STEEL slip-fit racing 1-5/8" (Off-Hwy only) Product Type HEADER (does not meet emissions requirements for street use)