A few times a year I need to replace a thin rubber gasket of some sort.

It's often quicker and easier and cheaper to cut a rubber gasket to fit
than it is to order the parts, especially since I'm 20 miles from the
nearest hardware store.

I used to use old car and truck tire tubes for scrap rubber, but they're
hard to come by nowadays I think (at least I haven't *seen* them in a
while.

What do *you* use for your scrap rubber gaskets?