You can find the updated list here: The Characters and Costumes Of The Simpsons Tapped Out
Last Updated: September 17th, 2014.
A useful trick in the game to find a character or to reset a character’s task is to put their associated building (or decoration) into storage. This cancels the task they were doing. When you place the building back in your Springfield, the character is released and is available for a new task assignment. Note of caution: this will also reset the building’s earnings. It’s not a big deal if it’s a building that earns often like a Quik-E-Mart or the Flanders House, but if it’s Burns Manor (which takes 48 hours) then you might want to reconsider.
There is another way to reset a character’s task. If they are working at a building (either inside or in front of the building), then if you place the building in storage it will reset the character. A good way to check is to tap on the building, if the character and their task is listed, then they are working at that building. But if they are wandering around your town doing their task, you have to find the building they are associated with.
With so many characters and buildings in the game, which buildings are associated with which characters? Here’s a handy chart. (Lisa, Homer and the Simpson House are special, in that the game doesn’t allow you to put them into storage.) “Earns Every” is how often the building earns $ and XP.