If you are still looking for reverse gears give Terry a call at T&B Trans in Washington state.
I've gotten new and used parts for two W56's and everything I received looked really good and at fair prices. He will have used parts for the reverse.
FYI, he will ask you for a measurement of the small piece on the reverse arm (small rectangular piece held on by a c-clip) that slides into the reverse idler gear. There are at least two different sizes, one that measures approx. .3800" and the other .3050", make sure you get the right one.