When I ran a store in British Columbia, there were two mouth sizes on the gallon jugs that I saw. Most took the 38mm screw cap that Gumjump mentioned. I never did find a decent cap for the larger mouth ones that Sirs mentioned so i had to recommend a #8 bung. FYI, a #6 bung fit the smaller ones.
BTW, I don't recommend a bung for longer storage.
Steve