Finding a company's suppliers and customers is often very difficult. Many companies do not wish to reveal this information. One resource that will help with this is Bloomberg. Six Bloomberg terminals can be found in room 1160 (Business Resource Center) of the CMU-Q Library, 1 terminal in room 2035.
The Bloomberg function for supply chain analysis is SPLC <GO> - this tracks a company's key customers and suppliers.
For example, to find Apple's suppliers, search by company ticker and function. Type: AAPL <Equity> SPLC <GO>.
See the screenshot below to see the various relationships:
