My SuperTigre's 20 years ago all came with gaskets like you seek. And the mufflers just wouldn't stay on unless we used the red threadlocker stuff. (I'd use blue now)

And, like recommended previously I picked up a small sheet at an auto parts store and it was the same stuff.

You can also take the back of a legal pad, the thicker paper some call kraftkote and make a gasket from it. You could also cover it with a thin skin of RTV if you worry about saturation although I don't ever remember that as a problem... the gunk helps maintain the seal.

Heck, if it doesn't work out like you wish you can always go metal-to-metal.

Regards,

Clay