Most (if not all) auto parts stores have it. Many here use it for everything that uses gasoline.

so it can do no harm so to speak...even in a gas / oil / Stabil mixture...?

Won't hurt a thing, and might even help.

Probably no harm or help.

I thought it was suppose to be good for cleaning things out...if not, what purpose does it serve...?

Sea Foam will dissolve carbon over a period of time. Excessive carbon can stick rings and it can flake off and short the spark plug . If your running rough I would look into checking the carburetor mixture and replace the spark plug.