Most companies will post their current and some previous annual reports on their corporate websites. AnnualReports.com provides the most recent annual report for most corporations in pdf and html versions.
The annual reports filed with the U.S. Securities & Exchange Commission (SEC) are available online in its EDGAR service.
Older annual reports are available in LexisNexis Academic:
Also in Mergent Online for last fifteen years.
Annual Reports at Academic Business Libraries -- See this website for individual collection details.