The U.S. Census Bureau is a great source for population, housing, economic, and geographic data. Its American FactFinder website can be difficult to search, so check out these tips. Here's some of the data available:
QuickFacts offer statistics on population, age, gender, ethnicity, homeownership, education, income, retail sales, and more.
The Pima County Public Library (PCPL) has a wealth of business-related information on its website. Some resources (like the useful Business Plans Handbooks) require a PCPL card for online access. Other resources, like business directories and business statistics and demographics, are free for anyone to access. To get a library card, visit any PCPL branch (the Joel Valdez Main Library and Himmel Library are closest to the UA campus).