A great place to start is the U.S. Census Bureau. You can get numbers from the 2010 Census, the more recent American Community Survey, and other data sets. 

• Use the American FactFinder to access Census Bureau data.

Many reference books at the Downcity Library include city-level data ranging from population to climate to salaries to quality of life.

• Cities Ranked & Rated
• America's Top Rated Cities

The library subscribes to several databases which are rich in demographic data.

• MarketResearch.com
• MRI+ Internet Reporter (you'll need to create your own account)
• SRDS (the section labeled Local Market Audience Analyst, on the left under Other SRDS Services)

See the Doing Market Research libguide for further help locating demographics.