Standard Industrial Classification System (SIC, 4-digit) Code Search –
https://www.osha.gov/pls/imis/sicsearch.html
SIC Division Structure (SIC Manual)–
https://www.osha.gov/pls/imis/sic_manual.html
North American Industry Classification System (NAICS, 6-digit)
Search and Manual –
https://www.census.gov/naics/
SIC and NAICS codes are standardized codes for identifying and classifying companies and employment in various industries. SIC is a 4-digit code developed initially in the U.S. in 1937 and subsequently used by other countries. It was supplanted by the more expansive 6-digit NAICS code in 1997.
SIC and NAICS help identify and group companies in the same business or industry through a single, standardized code. They will be instrumental when doing most searches in academic business databases.
You are welcome to start your company research by going straight to the source(s) listed above to find SIC and NAICS codes. That said, if you are starting entirely from scratch or not having luck on the official SIC and NAICS code sites, we recommend going to the Industry Surveys (Standard & Poor's NetAdvantage search) . . .