The easiest ways to find a business tax ID number, or employer identification number, are to browse through tax documents or contact the business in question. Some websites offer online databases with the tax ID numbers of many companies, but these services typically require a fee.
Some tax documents containing an employer's EIN are forms W2 and 1099. If you can't find these forms, contact the human resources department of the business. Companies are typically willing to give out this information, especially if you are a current or former employee.
If these methods are unavailable, you may still be able to avoid paying a fee if the business is publicly traded. These companies must file annual reports that contain the company's employer ID number, and annual reports must be made available to the public.
If you need the EIN of your own business, similar methods are necessary. Look for the confirmation letter you received when the IRS first granted the EIN. If that is not available, contact a bank with which you have a business account, and ask if they can locate the number. Tax documents for previous years may also contain the EIN. If all these methods fail, contact the IRS at the Business & Specialty Tax Line.