Whenever it is deemed necessary or desirable by any department head to make any purchase of supplies, material, equipment or contractual services in order to promote the more efficient operation of the public service of the agency under his direction, he shall prepare and submit to the Director of Finance a requisition therefor. Such requisition shall specify the nature of the purchase desired, the quantity required and the performance requirements to be met. It shall also indicate the date on which delivery is needed and may propose the names and addresses of potential suppliers.