I haven't played in 3.03 yet (as in since yesterday morning), so I can't speak for the current version of the game, but what I do when I need a rank up missions is go to a system aligned with my chosen superpower where as many factions as possible are also aligned, AND I have already become allied with all the factions. For Feds this is Sol (home sweet home), it only has 5 factoins, but they are all Fed and give huge numbers of missions. For Emp, I usually go back to Cemiess. I've never had a problem finding rank up missions this way, even if it does sometimes take me somewhat out of my way.
However, since continuing to work for a superpower after you've reached 100% of the current rank, will continue to clock up reputation in the background, I don't tend to worry too much if I don't see a rank up mission quickly. You will not 'lose' any progress ever. I'll keep farming for as long as I have the appetite, then when I finally get bored I'll go look for a rank up mission somewhere the superpower is extremely prevalent. One time, I did the navy rank and was still at 100%, and was able to get another straight away.
