Try storing and replacing Springfield Penitentiary.
Try going to Other Springfield and looking around near the Sprawlmart - do it quickly as Sideshow Bob will run away and hide.
Well if it was me who had the problem here's what I would do:
Store every character and building that had a character associated with it. Then put them out one by one in the order they appear in the game (here's the link to a guide right here in the forum - it's up at the top of the first page).
You might also try just putting everything in storage and rebuilding your town. I've read of cases where a glitch has allowed people to place objects where they shouldn't have been able to and gotten things messed up. As soon as the badly placed object was stored the issue was resolved. I do not advise nuking. I never advise nuking. I've seen too many complaints about bad things happening when people nuke. If you're going to put everything in storage (and I should note that not everything CAN be stored - Simpsons house, Heights crafting buildings, etc) then you should use the grab tool and store stuff by section. It takes a little longer, but it's a safer option than nuking.
Also, if there are some design features you want to keep you should take a few screen shots for reference when you rebuild. Having a visual reference makes it so much easier when you rebuild (personal experience talking here).
March 2017 - last edited March 2017
It is happening to me too. I have played through Tapped Out half a dozen times and always got the sideshow bob quest line very early on. Level 32 currently - no quests. It's broken, clearly, I think.
September 2017 - last edited September 2017
This has been an ongoing issue for a long time and unfortunately there is no one fix for this. I'd try all the suggestions posted but don't fret over it. Sometimes he just appears. It may not be for hundreds of levels though or you may get him right away. He's supposed to show up early in the game but he became glitched at some point and had never been repaired.