I am looking for a commercial input device(Button ) , which can interface with PC,preferably via USB, and serve as a substitute for a standard PC keyboard space bar press.
The closest thing i can think of to a universal keyboard emulator is a presentation remote, but the emulate page up and down, iirc.If you're willing to put a little elbow grease into it (and solder fumes), there's options.
You could use a teensy - a microcontroller thats often used for custom keyboards and keyboard emulation- with the appropriate button connected to the pins One of the examples covers the necessary hardware and firmware. Add a nice case and you're golden. This will work anywhere a keyboard would with no additional drivers needed.
You could also, presumably use a programmable mouse, and set one or more of the keys to spacebar only, though this is clunky.
Finally, if you can find one the griffin powermate might be an option - apparently its a programmable knob.