Call the company or visit the company's website. The TIN may be listed on the website (try the "About" page) or there may be documents on the site that contain the TIN. For example, invoices usually have TIN numbers printed on them.
Find a publicly traded company's SEC filings at the Securities and Exchange Commission's Edgar website. The TIN or EIN will be listed on the first page of any filings.
Obtain a company's IRS form 990, if they are a nonprofit organization. The TIN will be listed on the 990. You can obtain a copy of a company's 990 directly from the IRS.
Locate the TIN using an online database. For example, The Foundation Center keeps a record of nonprofit 990s for the last three to five years. You may also be able to find a for-profit company's information at KnowX.com or FEINSearch.com