Happy Trails makes them for their pannier racks.
Not sure, but they probably sell them seperately. Not much to them, just a piece of 8" long, 1" x 1/4" aluminum bar stock with a big hole in each end. Holes are 6 1/2" on center, which puts your signals about even with your tail light.
Their method is pretty simple. You could probly make your own in a few minutes once you see them.