I suspect no successful company outsourced their business plan originally. Do it yourself.
That's actually a damn good question. Going to a vc for funding without a solid business plan might kill your chances of securing funding. I would suggest probably having a look at how your peers in the industry have made theirs. You could take the common elements and build upon them as well as add your own bits. However i would seriously go against getting it outsourced since you know your business better than an outsider. What you could ask someone to do is develop a barebones ( wireframe like ) business plan.. Also you could have a look at business plan templates to get an idea of how your business plan should look...Few example that should help you out are
1. Microsoft templates 2. Business in a Box
You will find a range of consultants who will HELP you write a business plan, but I wouldn't trust anyone who is going to write it for you. Business Plan Consultants are usually helpful in that they ask all the tricky and unusual questions you wouldn't have thought of and force you to really think your idea through.
Especially when you are just starting out you knowing what to focus on, how to do the research, how much analysis to do in your cashflows, what the hidden extras are in cashflows are all things that someone with experience can contribute greatly to.
I wouldn't however just hand over to them saying "write a plan for how I beat facebook" and wait for the resulting 200 page plan.
As for the "online" component, I don't know of any online business plan online sites that are any good. This i think comes down to the fact that it takes a lot of working through, a lot of questions and a lot of refinement from both your idea and their understanding until you have something good ... this is hard when your not sitting face to face.