JMH,
You should be able to get the seal at any good parts place. If you remove it and take it with you, they will either find the number on it or pull out their calipers and find one for you. Don't go to a place that just has had kid with a computer, as he won' be able to help you, most likely.
For the diaphragm itself, once I located the part number, I simply called my local Chevy dealer parts department. When I gave them the number they came back with "that is an obsolete part, but let's look in our parts locator computer" A couple of minutes he came back on and said a GMC truck place in Phoenix had one, and sure enough, they did. I tried them again, when going over the differential itself, I found severely chipped spider gears. They, of course, didn't have them, but gave me several locations around the country that did. I was able to locate the gears and the corresponding thrust washers, as well. It is pretty cool when you are looking for 60 year old parts and someone went to the trouble to enter old stock into a national data base.
