I doubt any type of gasket (except woodstove gaskets material) would work with the heat on a fire box, which doesn't mix well with food safety.But I imagine somewhere they must make asbestos FREE gaskets.
But if you are looking for a gasket on the seperate cooker area, I would go to any appliance dealer.... walk their showroom and pick out what gasket fancys you on one of their more industrial type oven doors......... and order replacement parts for as much material as you need.
Unless you're running your cooker indoors, why would it matter if it leaked a little smoke here and there? My cooker leaks in places and doesn't seem to effect the cook. Close tolerences on your cutting and fitting can eliminate most if not all leaks. If you make your opening/door edges with a "S" shape so they close "into" each other, you shouldn't have much leakage. Smoke is like water.... it will choose the path of least resistance, hopefully your cooker exhaust.
You can put as many exhaust stacks as you want... and as big as you want for looks. Just be sure to be able to regulate or close them to match the needs of your cook.