KW Cages make well-built quality wire cages.Their cages are typically used by breeders, so they aren't as big asmany house bunny people prefer. NIC cages are nice, but it sounds likeit wouldn't work in your situation, as they aren't easy to transport.
If you got the largest wire cage you could manage and put it oncasters, as long as your bunnies get plenty of out of cage time, I amsure they would be very happy. If you are worried about the wire bottomon their feet, you can get grass mats, ceramic tile, or cage rests(also sold by KW, I think) to give them a place to get off the wire.