I'm not sure where to get the gasket anymore because I used to buy them off of Surfjetlouie (RIP) and Superjett-113 who doesn't sell them anymore.
However, if your bolts backed, chances are your threads in the manifold are getting weak and oversized and won't hold as well as they used to. I would try permanent loctite or JB weld on the threads when you get your new gasket. If they back out again, you will need to install helicoils or timeserts or you will forever be replacing gaskets and cutting your rides short. Trust me, I've been there.
Did you lose the bolts? If you still have them, you can pretty much guarentee they are still good and it's the manifold that caused the problem.