The USC Libraries has only one business database that provides a list of suppliers: Mergent Online.  Search the database for your company, click on the Horizon tab on the company's profile page, then click on the 'Suppliers' sub-section (see image below). This database is not 100% complete and does not cover all companies. Only publicly-traded companies are listed in Mergent Online (no private companies), and the Horizon supplier link is disabled for foreign companies.

You can also try these sources:

• Major suppliers of a company may be mentioned in press releases or in journal articles.
• If the company is public, and has a few very large suppliers, they may be mentioned in the company annual 10Ks. 

NOTE: It is very difficult to get a comprehensive list of suppliers. There can be hundreds of suppliers for any company, and any list you find may not be completely current.