Market share is often difficult to locate and it is usually easier to find market share data for consumer products such as soft drinks. You might be able to locate market share in the following resources, but market share won't be available for many companies/products.
The above strategy will not work for every product/company/industry. There is a chance that you will not be able to locate reliable market share data for your topic. You might have to resort to listing some of the key companies in the industry instead of providing detailed market share data.