The key competitors or major firms in a particular industry can be located in several ways. Just about any of the Business Directory databases will list key firms/competitors when a particular firm is searched. Such databases would include Mergent Intellect (essentially Dun & Bradstreet Hoover's profiles), Mergent Online, LexisNexis Company Dossier, ReferenceUSA and Standard & Poor's NetAdvantage. In addition, comparative market share or ranking information is often provided in these profiles and lists.
Within Standard & Poor's NetAdvantage, in addition to the competitor lists provided under each company profile, there are more detailed analyses within the "Industry Surveys" part of this database. A search under the "Industries" tab provides analyses of 50 different industry segments. These well-known and respected reports, revised every 6-9 months, contain both descriptive comparisons of major industry players and also statistical rankings by various financial rations (eps, net income, etc.).
Finally, one can also look in such a print source as Market Share Reporter (Ready Ref. HF 5410 .M35) for corporate competitor information.