If you know all of these except for Marketing, say, then this is where you will need to invest some time and effort. You can find a wealth of information by utilizing the traditional data sources such as chambers of commerce, major cities' websites, trade associations, the US Census Bureau, trade journals, magazine and online articles and advertising, etc. Performing keyword searches on Google, or Ask will bring up websites to check out. Following are some places to start:
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.
Small business consultants are very much on top of their game. They compile industry reports relating to the business, conduct feasibility studies, as well as studies relating to potential customers.
At this point, you may be tempted to skip writing a business plan altogether, viewing it as an unnecessary exercise in jumping-through-the-hoops, suggested by some old business professor who probably never held down a "real" job anyway.