Production Operation - If a product must be manufactured, what is the process? Will the work be done on-site or subcontracted? Who are the subcontractor(s)? If on-site, what space, equipment, machinery, production employees are needed? What suppliers are needed? Who are they? How will quality be assured? What is the anticipated production output? What established credit lines do you have? Possible Data Sources: local Chamber of Commerce; yellow pages; trade associations.
While these are easily available online, some of them are way better than the others. A lot depends on the approach that goes into writing the business plan.
Marketing - How is the product or service priced? How will it be distributed? How will it be promoted? Will it be promoted by the venture or an outside agency? What agency? How have you determined what amount to set aside for marketing? How have you determined product or service forecasts? Possible Data Sources: on-line searches; Amazon; local outlets; trade journals; industry attorneys & accountants; salespeople.
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: