The business section librarian can help. Major accounting firms also publish operating numbers for various industries. Your accountant may be able to help you get to the right accounting firm. Each industry's trade association has operating statistics. A list of all trade associations is catalogued in a good business library.
Many trade magazines publish industry statistics.
The more corroboration you obtain from various sources the better. Your business plan must identify the sources used. This broad information may not reflect the uniqueness of your product or service, specific selling techniques, the personality and size of your company. Therefore, you need to narrow it down by making it a better fit. Perhaps the trade association or the accounting firm can refer you to some of the actual companies in your industry that may be willing to share information with you.