Bloomberg offers a function for supply chain analysis, SPLC <GO> , that tracks a company's key customers, suppliers and competitors.
For example, to find Amazon's suppliers, search by the company ticker and function. You would type: AMZN <Equity> SPLC <Go>
View the relationship list in a chart or in tabular form.
Suppliers can be grouped by country to assess geographic concentration in the supply chain.
The supplier list can be filtered by industry subgroup or economic sectors.
Check the company's SEC filings--the 10-K, the 10-Q
EDGAR (SEC) All companies, foreign and domestic, are required to file registration statements, periodic reports, and other forms electronically through EDGAR (Electronic Data Gathering, Analysis, and Retrieval system). Its primary purpose is to increase the efficiency and fairness of the securities market for the benefit of investors, corporations, and the economy by accelerating the receipt, acceptance, dissemination, and analysis of time-sensitive corporate information filed with the agency. Anyone can access and download this information for free.
BWL subscription databases which contain links to SEC filings:
Hoover's Online Search for a company. Click on the "Financials" link (not all companies include "Financials")