You can always go down to the local beer store and inquire about 1/6th kegs which are about the same as a soda keg in size and volume. I know it's not home brew, but it'll still be on tap!
A few tips regarding the conversion that you may want to keep in mind whilst converting:
a) Before you drill the 3" hole in the top of the fridge, put at least two screws(recommend four) back in the plastic top to keep it from shifting while you drill. Trust me, it will shift.
b) No matter how neat it looks when it falls through, do NOT touch the metal disk that was once your fridge top directly after drilling. It will be a tad hot...
c) Once you hit the styrofoam, sloooooow down and don't press too hard or you'll crack the plastic liner of the interior fridge wall. When you hit the foam, it will take off!
d) You might want to let the silicone sealant dry overnight with the door open before plugging the fridge back in to allow it to air out. Unless of course you like the smell. It will linger inside a closed fridge for about a week.
Just wanted to pass on a couple things I learned...
Good luck. It's actually REALLY easy.
P.S. You can see my gutted house in the background. I don't have a house, but I have a keg fridge...