After I put on my M4 system I have my stock system to my buddy and he took my the stock header to auto mom/pop style exhaust shop and asked them to de-cat it.. I think they charged him $40 or $50 for "welding services"
Some of the chain shops might not be willing to do so as its illegal, but maybe you can talk them into it being for a offroad race bike??
They might be able to do so with the header on, but its really easy to take the header off, remove the belloypan, 4 bolts on the head, one bolt mid span and then the muffler as well.. Took maybe 5 minutes..