So I have some good news to share regarding gaskets for aftermarket header to aftermarket exhaust:
My S30 240Z has a modern day PS30 "432" style exhaust manufactured by TRUST.
It's twin 50mm pipes running the length of the car with a muffler at back with the vertically stacked beveled exhaust tips.
My C10 Hako has a "GT-R" style stainless exhaust.
It also features twin 50mm pipes running the length of the car.
The header to exhaust gasket is identical on both cars.
The header gasket that came with both is a metal coated cardboard type situation, which apparently is as easy to find here in SoCal as unicorns... I went to Midas, Pepboys, Autozone, 2 mom n pop parts stores, 2 mom n pop muffler shops and NONE of them had anything that came close to matching what I needed.
So I searched Yahoo Auctions Japan.
I found something with a Nissan part number that looked like it would work.
It was listed at 1500jpy (about $20)
I asked Adam @ FEAST to bid on it for me.
He did me one better, he called the local Nissan dealership parts department in Yokohama and found that not only was the part number still in production, but its retail price (750jpy) was cheaper than the YAJ auction link.
I had him order several and mail them to me.
I just test fit them and they work perfectly.
They are a nice 5 layer metal gasket - no paper of any kind.
I'm quite happy...