Brad,
If your customer isn't only interested in finding some NOS or excellent condition mild steel originals, then Dansk/JP are the current Porsche OEM for classic P-car exhaust, & they sell both the stock steel or SSI stainless flavors the last I saw. They may not continue the mild steel ones since the SS is more durable, & they can be prepp'd & painted Porsche Exhaust Gray, but they may still have some in stock &/or plans to continue production.
There are many Porsche parts houses selling Dansk, so you can shop for best price &/or sales/discounts for your customer.
You might also consider getting the Dansk/JP 2L Banana muffler & the exhaust hardware & gaskets at the same time.
AFAIK, all of the PCA Zones & Parade, & most of the other auto organizations/clubs concerned with Concours originality will consider the replacement parts from a current OEM for wear & deterioration parts as correct.
However, they may not be as accepted by some anal CWs buying collector cars for "investment" - but then, that type is usually looking for all original confirmed ultra-low mileage cars - & not restorations.
Good Luck!
Tom
///////