I came across this same situation and called up this guy that has been making unicycles for a living for the past twenty years.

from what he told me...there is no single unicycle supply source. Almost everything has to be fabricated by the builder when it comes to dropouts. He said he used to use pieces of pipe that was cut down to the desired thickness and then cut in half to make his unicycle dropouts.

at that point i decided that unicycle making was not in my future at all.

here is where I got the guys contact information...

http://www.tux.org/~bagleyd/unicycle...ry/custom.html

just a forewarning, hes quirky, but very friendly and helpful.