The Farmer's Friend did actually come with rubber scope end caps. These were to protect the ends of the scope, when it was stowed in the wooden case.
As for finding replacements... the chances of finding an original pair are pretty much zero. There were often lost and very few FFs on file still have them.
However, they literally are just plain rubber caps. I am sure you could find replacements that look just like the originals. Check places like Mc-Master-Carr online, or just start searching Google for places that make rubber caps.