Cut your own or get some of the fiber filled copper ones for aircraft. They work well, too. There's really no code to have to work from for gaskets.Bare
The Nfieldgear catalog gives the torque requirements for the two nuts as 4 foot/pounds.That isn't very much torque but it will produce about 750 pounds (340Kg) of clamping force at each stud. That's a total of 1500 pounds (680 Kg) of force holding the exhaust in place.If too much torque is applied to the nuts in an effort to seal the joint, the studs may be broken off or worse, pulled out of the aluminum cylinder head.Go easy with the torque. A leaking joint is a lot better than a pulled out stud.