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Be sure you are researching the official name of the company. Many companies have popular names that may not be listed anywhere. For example, 3M is really Minnesota Mining and Manufacturing.
If the company is a personal name or includes initials, e.g. Walt Disney or H. J. Heinz, look under each part of the name. Various sources are not consistent in how names are treated.
If you are researching a subsidiary, expand your research to include the parent corporation. For example, Google is under its parent company Alphabet.
Companies sometimes change their names, so be sure to look under multiple possible company names. For example, the cigarette maker Phillip Morris changed its name to Altria .
Very small or very new companies may not appear in the standard sources, particularly print sources. Look for news articles and a company website. Use any regional or specialized industry directories which may be available.