Company/Product rankings are commonly available in such sources as Standard & Poor's Industry Surveys (see NetAdvantage), or Market Share Reporter (HF 5410 .M35 Latest ed. in Ready Reference, earlier in Book Collection)
Rankings are another way of looking at company comparisons than just market share. Quite often, rankings by sales are more commonly provided than a strict percentage market share. Or there might be a ranking on some other basis, like tonnage output in the steel industry. You also may be dealing with a firm that is highly diversified in its lines of business or products, in which case you’ll have to look at rankings or market share for particular business segments. Some industries themselves, like apparel retailers or restaurants, are extremely segmented, and this fact often makes the “market share” question much more elusive, or expensive!
Some popular or well-known rankings lists include:
Fortune 500 (also available in annual spring, print issue of Fortune)
Global 500 from Financial Times
For additional suggested sources and links, see U.S. Library of Congress's Rankings/Lists: Doing Company Research.