Everyone may use.
Available free through the U.S. Census.
Link to Database
Zip Code Business Patterns provides statistical information by NAICS code, within zip codes, from 1998- present (usually 2 years in arrears) including:
- Total Number of Employees
- First Quarter Payroll
- Annual Payroll
- Total Number of Establishments
"Number of Establishments" is further broken down by several employee size classes. The Census started using NAICS codes in 1998 so prior to 1998 you will find data using SIC Codes on the Census web site.