FAR 7.102 (a) states "Agencies shall perform acquisition planning and conduct market research (see part  10) for all acquisitions in order to promote and provide for — (4) Appropriate consideration of the use of pre-existing contracts, including interagency and intra-agency contracts, to fulfill the requirement, before awarding new contracts. (See 8.002 through 8.004 and subpart  17.5)."  Please note the quoted reference also refers you to FAR 8.002-8.004 and subpart 17.5, so you should review these references as well.  In addition to FAR 8.002 - 8.004 and subpart 17.5, also take a look at DFARS 208.002, DFARS Subpart 208.70, DFARS Subpart 217.5 and your agency regulation.  DFARS Subpart 208.70 covers the policy and procedures for acquisition of items through another department/agency within DOD or through GSA when a department/agency or GSA has been assigned contracting authority for that particular item.