If you can find a copy of "Brew Ware" (I have it checked out of the library), it has details on how to build your own.
The theory is: You have three lines and three valves. One goes to your CO2 tank, one goes to the fluid out on the keg, and one goes to an overflow bucket.
You open the CO2 valve to equalize pressure with the keg, then open the fluid valve to allow beer to fill the bottle. You close the fluid and CO2 valves, then open over flow valve and remove the rig from the bottle.
If you break the seal before you release the pressure with the overflow, you get a shower.