Concours Parts in Carson City, NV shows rear shocks for your ’56 on their website. They advertise in Quicksilver and their info is on page 49 of the winter issue. C&G Early Ford parts in Escondido, CA also shows they have shocks that fit your ’56. Best news of all is that NAPA Auto Parts also carries these shocks. Hopefully you have a NAPA store close to you so you can save on shipping.
I’m not so sure that the shocks are the issue for your bottoming out. Shocks are there to dampen the travel of the springs so that the car does not bounce continuously after hitting a bump. They are not designed to affect ride height unless of course you are using air shocks. I have ridden in a car with no rear shocks in place and it is a very scary ride. My ’55 also has issues with being “too low” in the back. It probably wouldn’t hurt to add one more leaf spring. That worked wonders on the ’53 I used to own.