Bloomberg (see access details) has data (when available) on a company's suppliers and customers.

• Use the SPLC function for a specific company -- e.g., to find Merck's suppliers, customers, and peers you'd type: MRK <Equity> SPLC <GO>.

S&P Capital IQ (see access details) also has supplier data. 

• Search by company name.
• From the left-hand menu, locate Business Relationships and select Suppliers.

Use ABI/INFORM Global and Business Source Complete for articles from trade publications which may mention a company's supply chain

• Try searching for supply chain AND [your company's name].