R & M Water Group, S N 78950

We recently purchased a home that had a water softener made by R & M Water Group, Serial Number 78950. It has a slow small leak where the valves attach to the top of the black round cylinder, not the salt bin. I believe that there is an "O ring" where they join and it appears to be leaking. How much of a problem is it to replace this "O ring" and where can I obtain the correct replacement? You help concerning this matter will certainly be appreciated, thanks.