First you get a good rugged gi to stuff. Then you walk the streets at midnight, and the first chump that decides to mug you is the lucky new employee to fill out your "dummy". If there's more than one guy, then you can make different sized dummies, or maybe sell the extras. After all, nothing can better imitate the feel of a human body besides...well a human body.
I've recently wondered about anyone making their own grappling dummy. I figured you would always have to buy one. I'm very new to the ground game, so I'll just sit back and watch the responses.