There is the option to make new tanks too, if you can find the similar size food cans*, a screw top from readily available tins could be cut out and soldered in.
The original O&R tanks are probably made in the same way as early (steel) beer cans were made in the US, I've seen a few in some of the abandoned mine exploration videos I watch on youtube and they looked similar in size & construction.
*I've saved a couple of coffee cans for experimentation.
David