Where to Find Historical Rental Prices?

Is there anywhere I can get a chart of historical rental prices over the last 10 or more years by a region, city, or zipcode? Preferably month-by-month, quarterly, but yearly would work as well.

Rent's are up in many of the regions I'm buying. In some places I have seen as much as a 50% rent hike in the last 5 months on properties I've owned for years. I'm looking for a barometer to see where the floor is for the property we are making offers on.