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.
One key component critical to your success is scoping the competition. A business plan consultant studies the area where you will be located quite thoroughly. For example, if you plan on setting up a coffee shop on Main Street, or within the vicinity, the small business consultants you consult with will inquire about the number of existing coffee shops and the services they provide.
Marketing - How is the product or service priced? How will it be distributed? How will it be promoted? Will it be promoted by the venture or an outside agency? What agency? How have you determined what amount to set aside for marketing? How have you determined product or service forecasts? Possible Data Sources: on-line searches; Amazon; local outlets; trade journals; industry attorneys & accountants; salespeople.
"Plans are useless; planning is indispensable," according to Dwight D. Eisenhower, then Commander of the Allied Forces in Europe during WWII. Now, you may be in total agreement with the first part of that statement, but you are really not convinced of the truth of the second part.