An easy way to find competitors is by industry classification, which groups companies according to their business activity. Use these NAICS codes to search for other companies competing in the same industry.
Also consult the Library's Company Research guide: libguides.bentley.edu/companyresearch
ReferenceUSA (now known as Data Axle) contains detailed information on U.S. businesses. ReferenceUSA records contain contact information, corporation information, company descriptions, management directories, and competitors. The custom search feature in ReferenceUSA allows users to set up a personalized search form to help identify and compare companies. Custom search options allow users to limit search results by business type, geography, business size and ownership type.
After launching ReferenceUSA, click on the link for "US Businesses" and then the tab for "Advanced Search".
Reference USA lets you download/export up to 500 company records at a time into excel. You can slice the data even further by looking at the other options available in the Advanced Search, just remember to Update Count when you add criteria before View Results.
Please remember that brand names and company names are not always the same and that not all of these companies are competitors.