Ken, I got them from ebay is the US. They come in red, blue and yellow. I think i also saw green too.
http://stores.ebay.com.au/PEACHNMINI-BIKE-STORE/GRIP-/_i.html?_fsub=2402066017&_sid=148114587&_trksid=p4634.c0.m322
...s...
These operate from the trikes electrical system:
http://www.ebay.com.au/itm/GERBING-G-12-12-VO...hile since they've had the DC power packs available).
http://www.ebay.com.au/itm/GERBING-HYBRID-12v...
I found this link: http://www.ebay.com/itm/4-Shepard-12-Super-Tri-Dollies-3-Wheel-Heavy-Furniture-Moving-300-Lb-Ea-Bonus-/271159289257
But I think the wheels/castor need to be bigger. These won't handle the bumps so well. Depends on you ground surface i guess.
Also found a discussion thread on the very same airplane. O the things you can find on Ebay
http://www.homebuiltairplanes.com/forums/hangar-flying/7134-cool-plane-flying-saucer-u-f-o-experimental-aircraft.html