You can find a variety of companies online to help you with your market research. For example: Sundale Research's (sundaleresearch.com) primary goal is to provide new and mature businesses with objective, accurate industry data and market analysis on a wide range of topics. Their market research is intended to save you time and money while keeping up with industry trends. 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.
When writing a business plan you sometimes end up locking yourself away. You might have unique ideas which lead you to seek some isolation and secrecy, or if you are going to be a sole trader you may only have one person to consult namely yourself. But it is fantastic to try and get broader input on your business plan _ whether from a professional, or simply from friends and family whom you trust. I say that because of course you need to be careful with commercially sensitive ideas, as you do not want to pass your plan on to someone in the pub who then starts your idea before you across the road.