Gardener's Supply makes a 58 gallon self-watering raised bed for $169:
- 39-1/4" x 39-1/4" x 16" H
- 8-gallon water reservoir
- Holds 9 cubic feet (230 dry quarts) of Container Mix
- 12" planting depth
I don't have one, but I've seriously considered it.
You can add 16 casters (for $40) to it to make it movable, but I imagine it's still pretty heavy.