The origonal wheels are available from Dualpumps ( the one with grease nipple on top) but thier crap. If you ask them for the better ones they supply a much better castor assy without grease nipples.
You can't really put bigger one on as they will bind on the side of the plastic hood, and if you could larger wheels would lift the nozzle height further off the floor resulting in less pressure and you will get more spray from around hood which undermines it's origonal purpose
