You're not going to find one in any auto parts store, your best bet is to:
A, keep an eye on ebay if you don't need your car up and running real soon.
B, post a wanted add in the forum(close to the bottom of the main page)
C, buy a Fel-Pro or Victor head gasket, caution: they're about .025" thicker than NOS head gasket so you lose compression if you don't shave the head to compensate)
Good luck,
Stephen