In the UK, Maplin Electronics sell these things, or at least they used to. A few years ago Patrick (empeg hardware designer) and myself worked with a company developing immersive simulators and we tried these little subs under the seats (which were fibreglass Sega seats at the time). They didn't work very well - "rattled" a lot I seem to recall.

The last time I saw the project they had uprated to 10" drivers in a ported enclosure right behind the player's back (and replaced the horrible Sega seat with a proper rally car seat). I don't think they need hydraulics to make the thing shake now :-)

Rob