The installation files for Odoo Enterprise (9.0e) are only available after you buy the licenses. You have to buy a minimum of 5 licenses @ US$240.00/user (1,200.00 total). 

You can buy it from here https://www.odoo.com/buy (directly from Odoo) or find a Partner at https://www.odoo.com/partners

Also consider Odoo Online to see if it will be cheaper for you according to your needs. 

Compare editions at https://www.odoo.com/editions