Military radios in the 1940s and 1950s still relied on Valve technology rather than much smaller and less fragile transistors. These valves could burn out and would need to be replaced in order to keep the radio functioning and so radio operators carried spares in sturdy metal cases to prevent damage to the delicate glass valves. One radio in use during the early 1950s was the R209 Reception set, this receiver being designed late in World War II and used with the WS53 transmitter, and to accompany this receiver a specialist tin for spare valves was issued:
This tin is painted in the standard green finish, with white lettering indicating that it is a No 16 valve tin:
The tin is secured with a webbing strap with a buckle that is riveted to the tin and the lid:
This means that when the tin is opened, there is no danger of the lid getting lost or missing:
The original liner is missing from this example, but originally it would have had a rubber or sorbo pad with cut outs to hold the spare valves. The tin held: 7 valves, fuse, MES lamp and 12 volt non-synchronous vibrator. A search online has found an example with its contents, showing how it was laid out:
Tales from the Supply Depot 
I’m quite sure you know much more about finding kit online than I do, but a cursory search gave several sources for tubes/valves including the ones listed, the first one on the page happened to be in the UK too 🙂 https://mullard.org/collections/radio-valves-01a-to-4z
I don’t know if your equivalent of a Radio Shack or other electronics shop might have them instock for walkin purchase, but once upon a time every department store here had a tube tester and stock of tubes prominently displayed, right up to the 70’s and beyond. Some probably still have them gathering dust in the storage room.