My trx 300ex's rear shock is sagging over an inch now, so I'm assuming that means it needs to be recharged with nitrogen? If so, where can I get some and how do I go about doing it, or would it be best taking it to a shop? Or is this not even the problem and I should get a whole new shock and everything?? I'm not really sure, I've never done anything like this before.

This is common misconception you're presenting here. Shock sag has nothing to do with nitrogen pressure. Either you have impromerly set sag, wrong spring stiffness or spring is broken. Nitrogen pressure affects only cavitation threshold.