There is no single resource for market share information.
Market share is calculated by taking the company's sales over the period and dividing it by the total sales in a given market or industry.
This calculation is almost always determined by consulting various sources. Listed above are the best bets which you should consult immediately. For more resources, consider:
Articles in Journals, Magazines & Newspapers
Trade Associations & Trade Magazines/Journals
Annual Reports, Filings & Other Company Documents
For more on consumer data, industry reports, or general inquiries related to "market share" or business data, contact Benjamin Hall (see left).