Sorry can't help with spare gaskets, but for what it's worth I did make up some side marker light gaskets from some thin sheet neoprene that I was happy with and where easy to make up.
Heater box housing the one in the engine bay I believe is an adhesive and not a gasket, I used some strip rubber tape as a gasket to seal mine back together, though a proper one piece gasket would have been nice.
Taillight gaskets are like a molded piece if I'm not mistaken and probably tough to reproduce, could use some new ones myself.
Hopefully another member here can help take this further.
1964 Chevy K20 Panel
1973 GT Vega Hatchback
1984 944 porsche
1992 Lexus SC 400