To use some sources you will need a NAICS or SIC code for your industry.
North American Industry Classification System
The NAIC System assigns numerical codes to industries from two to six digits. Shorter codes represent broader industry sectors. Be sure to read the full description of the NAICS code, as it may include a broader array of products/services than you might assume.
Standard Industrial Classification Manual (at OSHA site)
The SIC system assigns numerical codes to industries from two to four digits. Shorter codes represent broader industry sectors. Be sure to read the full description of the SIC code, as it often includes a broader array of products/services than you might assume.
Market Share info may be found in sources listed under Industry Overviews, such as Mintel. Other ways to find market share are listed here. Note that different sources may provide different market share data due to varying definitions of the market or different methods for gathering and analyzing data.
The industry reports on the first page of this guide may also include market share information. Public companies may discuss their market share in Annual Reports or SEC Filings such as the 10-K.