Google can also be very useful for biographical information. However, it is important to Google WISELY. Remember the ABCs of website evaluation: authority, bias, and currency. If you are not sure if a website is legitmate or cite-able, just ask a librarian!
Did your person create or found an organization? Perhaps that organization has a page dedicated to that person and their history, like the Red Cross does for Clara Barton.
Museums can also be great places to go, even if your person is not an artist. Consider the African American History Museum or the Smithsonian. City museums like the Museum of the City of New York can also be useful, as can "niche" museums like National Cowgirl Museum and Hall of Fame.