If this is just the rubber friction ring, I think that the part is not just "similar to an o-ring", it actually IS an o-ring. Find the right size at the hardware store and you should be fine.

Or are you missing the whole adjuster?

Not sure I understand the problem with your chain. Is the "middle gear" the middle chainwheel (front) or a middle cog (rear)? If its the rear, first make sure that the derailleur hanger is true and not bent or distorted. If this is indexed shifting, the key is to get the zero-tension state (small chainring or cog) of the offending derailleur correct. And make sure your shifter cable is intact (not wearing way or partially broken) and make sure it's tight.

Beyond that: I'm a little rusty with the mechanics problems, too. I am sure that others here are more expert and can help you with more certainty.