Most auto supply stores have it. It is like a cereal box thickness of carboard, so you can cut it with scissors or a knife.
If you put a thin layer of oil or grease on the part, then press it hard against the gasket paper, it will leave an oil mark the exact shape you need.
Just cut around the oil mark, and you have made a new gasket.
Gasket paper is oil proof/resistant so it won't hurt it, but you could trace it with a pencil, or use wet paint, etc.
I have also used a hammer, and put the gasket paper on the part, and lightly tapped the hammer to break the gasket paper at the 90 degree edge of the part. Tap all the way around an item, and at each hole, and you have made a gasket by just holding it on the part.