Investment services provide major reference sources for finding company information. These sources provide detailed information about companies and are used in real-world business research for making investment and other business decisions.
Note: WRDS is a subscription provided by the Dhillon School of Business to DSB students and faculty only. New DSB users must create an account for WRDS here.
... to search article databases for specific company information, too!
Many of the databases mentioned here provide a list of competitors, but it's likely imperfect, generated based on NAICS or SIC codes. If you have a suspicion that the competitors are incomplete or incorrect, go with your gut and find other ways to discover them, based on your own familiarity with the industry, and comparing other companies by sales/revenue or market share. You can also find more like-businesses in trade & business news publications, tradeshow exhibitor directories, membership directories, and business to business sites.
(thanks to Business Reference and Services Section of the American Library Association)