U.S. Census Bureau (census.gov): A source for a variety of useful statistics, especially the Economic Census that comes out every 5 years.
Not surprisingly all these individuals have been in the business game for a long period and understand what they're speaking about. While you are in the beginning stages which may be just what exactly you will want, a professional available to assist you to on your way.
Apart from this, they also analyze consumer trends and gather legislation reports and regulations that might have a future impact on your particular venture. These expert individuals make it their responsibility to build upon previous experiences and the requirements needed for you to carry out your intended plan.
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.