One crucial function they perform is the formulation of business plans. This plan is based on the information provided by the client coupled with the vast wealth of knowledge and research a business plan consultant has.
American Demographics (adage.com/americandemographics): Just as the title suggests, numerous free reports about consumer demographics in the U.S. nationally and by statistical area.
You may find that the easiest part is the actual writing of the plan. The real work comes in the data-gathering, which may take you a hundred hours or more, depending on what you already know or have researched. If your new venture is in an area where you've been working, you may already know about your customers, your suppliers, your marketing plan, your organizational structure, your financial and cash flow needs, equipment, inventory, and so on.
Internet Public Library - The Census Data and Demographics (ipl.org)/: An especially useful site that has links to information about countries other than the U.S.