I'm thinking of getting some "non-disposable" expanding foam for
various insulation & sealing jobs. I've read the wiki article
http://wiki.diyfaq.org.uk/index.php/Expanding_foam
but I have a few extra questions.

Are the cans & guns standardized & interchangeable, or is it important
to pick out a "reputable name" of gun to ensure a long-term supply of
foam for it?

The wiki says:

When using a gun, the gun can be left on the can for short periods
of disuse but must be taken off and cleaned if left for longer
periods. Cans of cleaner designed to fit the guns are available.

Do the foam cans seal themselves well (like gas cylinders) when taken
off the gun? Do you leave a can of cleaner on the gun when it's not
in use, or just blow cleaner through the gun & then leave it open?

Thanks.