Another old trick is to simply find an old keyboard, rip the keys off it you don't need (all but space bar) and use that as a foot pedal. Just step on the spacebar to stop/start.
Works best on a laptop type system, where you can still use the internal keyboard yet have the external (stripped) keyboard function as foot pedal. You can't 'stomp' on it for obvious reasons, but it would work and be real inexpensive.

It ain't pretty, but if all you need is a space bar for your foot ..


I do not work here, but the benefits are still awesome
Make your sound your own!