It is possible to look up a federal tax ID number easily. You can use Electronic Data Gathering, Analysis, and Retrieval system, known as EDGAR. This system is maintained by the United States Securities and Exchange Commission (SEC) and tracks all information filed by for profit companies with the organization/ The system is available for as a free service online. Several forms contain the federal tax id number, or EIN such as the 8-K, 10-K and 10-Q reports.
If the business is a non-profit organization, Melissa Data is an organization working to provide important information about non-profits to individuals. One of the free databases available to the public is online and allows people to search for business information based on the name, zip code, or the tax ID. When searching, the results will render a federal tax ID number.
If possible, ask the business for their federal tax identification number. Someone in accounting should be able to provide the information. When using EDGAR, remember to try searching just the first few letters of the business name because not all businesses are listed under the name they are most well known by. When using Melissa Data, it is important to remember to use a zip code of a corporate office rather than a local branch.