Most industry profiles in databases have information on market size. You can also use the Economic Census data.
It is not always easy to find market size information on a very narrow or specific industry/market. Look for mentions of it in articles or on the web. When all else fails, you may have to make an educated guess based on existing information.
If you don't find market share information in secondary sources such as databases and websites, caculate an estimate:
Company's sales (if your company has only one type of product) or your product's sales (if your company has multiple products)
industry revenue (available in the Economic Census) or market size (available in industry profiles).
You might find that different sources may have slightly (or very) different "market share" data. If you are comparing market shares, use numbers from the same source if possible. Numbers are different because of