They are required in laboratories, hospitals and such so you could check out either an industrial supply or laboratory supply company.
You could probably also make one yourself, if you are even remotely handy....
If I were to do it...I'd probably take a 2x6 or 2x8 about 12" long and cut a semi-circle out of one side.. On the edge of the piece where I cut the semi-circle, I'd drill and glue in two bolts, and attach a chain to one of the bolts. The tank fits into the semi-circle, and the chain goes around the tank from one side to the other and attaches to the other bolt. Use wing nuts to make the chain removable. Attach piece to side of kegerator.
Hope that was clear.