There have been many questions about buying a 6v battery.
Batteries are identified by "Group Number" in the battery industry. Group number identifies the size of the battery and the location of the terminals. The 6 volt battery used in the Cub is the same as the old VW Bug 6 volt battery and they are still available as they are used in golf carts, floor machines, signs and many commercial applications. Ask for Group 19L or Group 1 and you will get a 6 volt battery that fits. You may then get choices on battery quality and the best measure of this is cold cranking amps or CCA. The higher the number the more plates in the battery and that means you have more "power" available especially in cold weather when battery capacity drops. You can buy batteries at Sears, NAPA, or any other battery retailer that will order a battery for you if they do not carry it on the shelf. Interstate Battery has them in their on-line catalog under vintage cars. Use VW as the lookup in their battery finder, but you need to look it up in the vintage car section to find it.
I have seen some 12 volt tractors with the wrong battery too. I think a lot of battery box covers and fronts are lost when the PO came home with a battery that was not the right size so the covers were set aside and eventually lost. The correct 12 volt battery is Group 26R.