Public companies are required to file certain documents with the Securities and Exchange Commission (SEC). Use the SEC's EDGAR system to search by company name or ticker symbol for filings. You can filter your results to find filings of a particular form type, for example 10-K (Annual Report) or 424 (Prospectus).
EDGAR filings are generally available from 1994 onward. Earlier filings may be obtainable through SEC FOIA requests and fees may apply.