I'd try a Napa or an old independent part house that still has the paper catalogs on a rack above the parts counter.
someone is going to have to chime in here because I don't know if that is a C clip rear end or if the bearing retainer holds the axle in.
Is there a large hole in the flange on the axle beween two lug bolts that you can put a socket through to reach the bolts that hold the backing plate on. And does it look like there is a flange bolted to the front of the backing plate?
I've got a 55 rear axle at the house but have never had the brake drums off and it's been too long since I worked on one to remember.
If it is 3100 with a stock rear axle this should be the seal.
http://www.napaonline.com/Catalog/Ca...695_0121701481
Bottom right corner
http://www.lmctruck.com/icatalog/ca/full.aspx?Page=85
It looks like it has a C clip axle which mean that you will have to pull the cover, take the bolt out that holds the spyder pin and take the pin out and then push the axle in so the clip comes out then pull the axle out.