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.
But your idea may be so new that you may also need to talk to potential customers, host some focus groups, talk to an ad agency, or maybe even make a prototype and float it past some people. Be prepared to spend the time. Remember, it's not about the Plan but the Planning.
Executive Summary - One to three pages that summarize all the information to follow; come back and write this last.
"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.