For when you know a brand and not the owner (i.e. Vitaminwater, a product manufactured by Energy Brands Inc., a subsiderary of Coca Cola):
WARNING: Keep in mind that all of the information that you want may not be found at the company web site. Big corporations pay fancy consultants lots of money to complete market research; often, companies aren't willing to share the information that has cost so much to obtain.
Trade Associations are a good way to find competitor websites.
Most of them have public member lists.
Using Google, search "trade association" and the words from the NAICS codes.
NAICS Codes (North American Industrial Classification System); R338.02012 N864ic