Description - What product(s) or service(s) are you offering specifically? Are any patents, copyrights, or trademarks needed? Have they been acquired/filed? What is the size of your business? Where will it be located? Will this require purchasing or building a facility? Will this require leasing a facility? At what cost? Has a lease been negotiated? What personnel will you need? Where will you find suitable employees? What equipment do you need? Will it be purchased or leased? What are the qualifications of your principals? How do their backgrounds promote the success of this venture? Why do they think this will be a successful venture? Possible Data Sources: local Chamber of Commerce; community colleges & local universities; local employee leasing company; real estate agents; US Patent & Trademark Office; US Copyright Office.
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.
U.S. Census Bureau (census.gov): A source for a variety of useful statistics, especially the Economic Census that comes out every 5 years.
Appendix - Resumes of principals/management; letters of recommendation from current business associates/customers/suppliers; marketing research data; demographic data; leases or contracts in place or as promised; business licenses; price lists from suppliers; trade or industry articles or data; floor plans; information on subcontractors; liability insurance policies.