You have to be very creative in your approach to recruiting to ensure a diverse workforce. Ask new hires for their help. Establish a relationship with an educational institution. Make use of the career placement office as well as the academic department of interest to you. Maintain a relationship with sources you identify even when you don't have positions to offer.
Nothing replaces the personal touch.
Use progression planning to identify people on your staff who may be approaching retirement. You may want to consider developing a co-op student as a replacement. Establish a mid-level intern program. Review your staff to determine whether you have candidates who could be developed to fill professional positions through use of a developmental program. Federal employment offers a variety of benefits that make working for the government attractive.