I found what I was looking for - a source. There is a local business that does most of the hydrotesting for the local business community. They of course see tanks that fail, so the guys who make bells buy scrap tanks there. They have lots of old caps. As it happens I have a WWII-era 100-lb propane cylinder which is heavily made and galvanized so it has no rust. Plus it has threads for a cap - its previous owner thought it was a huge acetylene cylinder. I use cylinders like that for the propane burners in meat smokers, so I decided I'm going to go get it hydrotested. They'll just throw in the cap.
I know I could just swap it without a cap. Around here we have 3 main welding supply stores: Airgas, Praxair and Central Welding. The one with the cheapest price on acetylene (Praxair) also is the one who charges the most for missing caps. Sigh.
Some of the people posting on this thread seem to be under the impression that there are many random threads for cylinder caps. Not true. High pressure cylinders use 3-1/8" caps, and they come in 3.5-7 or 3-11 threads. Fuel gas cylinders use 3-1/2" caps, and they come in 3.5-8 or 3.5-11 threads. So for each size there are two threads. Not many, not random, just coarse or fine.
metalmagpie