Information may be found in news, business or trade publications, company or industry profiles, annual reports, SEC filings, investment or analyst reports, consulting firms information and from industry experts.
Use NAICS (North American Industrial Classification System, pronounced "nakes") industry codes to find market size, develop lists of competitors, suppliers or customers, find industry ratios, and more. Many commercial publications use NAICS; some still use the older SIC (Standard Industrical Code) codes. When researching industry data, NAICS are extremely helpful.
See the US Census Business & Industry site for reports that measure industries using the NAICS.
Various state and federal governmental organizations track statistical information industries. The NAICS code can assist you in data collection for research by industry.