Who are a company's actual (not potential, or possible) suppliers and customers?  Sometimes this is easy to find, sometimes not. 

Problems you may encounter:

• Private company:  if you're dealing with a private company, remember that you'll always find less information. 
• Proprietary information (i.e. secret).  Often companies consider the names of their business partners to be sensitive, proprietary information, and do not disclose them.  Public companies must disclose the information if their relationship with a supplier or customer is potentially risky. 
• Too many and too diverse to list.

For more about finding potential customers, have a look at the marketing research guide.

Mergent Online has a Supply Chain content module.  For public companies (both US and international), it extracts information from public company documents, the trade literature and other sources, and creates links between the companies for:  

  –  Industries & sectors
  –  Products & services
  –  Competitors
  –  Customers
  –  Suppliers
  –  Partners
  –  M&A scenario report

These are generally lists of the major customers and suppliers, not comprehensive lists. 

By the way, usually it's well worth while to look at the entries for companies on both sides of the relationship; you can often get more information that way.

To get to the Supply Chain content: 

First do a company search!  Then look for Supply Chain tab (near the right end) below the company's name.

Public companies are required to name the companies that they deal with if, and only if, the relationships with those companies may place the reporting company in jeopardy.  For example, the reporting company would be in jeopardy if one of their major suppliers, or customers, were having problems or trouble.  This is usually discussed in the 10-K report in the Management's Discussion & Analysis section.  If the partner company gets into trouble mid-year, it may also be reported in an 8-K report.

There are several resources you can use to do full-text searching within filings. Below are links to two of them, EDGAR and Mergent Online. In the box below you'll find more information about ThomsonOne.com, the most comprehensive source.  It has probably the longest run of full-text filings, starting most comprehensively in 1996, but with a significant number also online long before that. 

In the end, what you most likely will have to do is scour and skim through the trade magazines for the industry in which your company operates.  Often the trade mags are where you'll find reporting on new contracts, lost contracts, joint ventures being considered, and so on. 

When searching, be sure that you're searching in the full-text of articles.  Use lots of keywords, truncation, and proximity command searching. If you're unfamiliar with the search commands for a particular article database, check their help!