One way to write a business plan is to hire someone to write it for you. It should be a somewhat long and interactive process, as this person must learn exactly what you're trying to accomplish, and how.
If you do this, be careful and thorough when looking for someone to do this. Ask for samples of their work. If they say it's confidential, tell them you'll be happy to sign a Non-Disclosure Agreement (NDA), and that the person can block out any sensitive information such as numbers or product design.
I've been hired a number of times to write a business plan, or to contribute only one portion. However each and every time it was by word of mouth, not because I advertised. So I'd say the best way to do this is through speaking with people you know and trust to help find someone.