5 C.F.R. § 300.403
When commercial recruiting firms and nonprofit employment services may be used
An agency may use a commercial recruiting firm and/or a nonprofit employment service in recruiting for vacancies when:
(a) The agency head or designee determines that such use is likely to provide well-qualified candidates who would otherwise not be available or that well-qualified candidates are in short supply;
(b) The agency has provided vacancy notices to appropriate State Employment Service and OPM offices; and
(c) The agency continues its own recruiting efforts.