69 Barracuda Gas Cap Gasket Seal

In restoring my 69 Barracuda I reused my old gas cap which has an old, dried out, cracked rubber gasket to seal it against the fuel filler neck. Does anyone out there know of a place that sells a replacement gasket/seal? Or does anyone have a remedy to soften and restore (as best as possible) the original one. I think I'm getting a bad seal which is allowing the fuel to partially evaporate. I don't mind getting bad MPG's while I'm driving the car but I'd rather not lose fuel while it sits.

Thanks everyone.