I am not sure if anyone sells those but I wanted to offer a way to make those flexible foam rubber maggots move.
If you are going to use them on a surface, then install a vibrating motor on the under side.
Another option might be the vibrating speakers from my other forum post. Basically, using an audio speaker the maggots should not only move but crawl over one another too. I would be very interested in seeing this option if you use it.