Do you sell your waterguns?

I do not. :-(
But the companies above do.

Where can I get a realistic motorized watergun?

Due to stricter laws the black tinted ones are not made any more. You can try to get one second hand on sites like Ebay. Entertech and Larami made nice ones.

I am looking for a powerfull portable hand pumped water cannon?

Just go to:

http://www.waterblaster.net

Who invented the Supersoaker?

Lonnie Johnson.

http://www.johnsonrd.com/ie/lj/ljprofile.html

http://www.isoaker.com/Info/history_supersoaker.html

How do waterguns work?