These websites consistently list nonprofit and foundation position openings. Some provide career counseling tips and in-person networking opportunities.
2021 Guide to Nonprofit Jobs & Careers
Association of Fundraising Professionals – Greater Philadelphia Chapter
Association of Fundraising Professionals – Brandywine Chapter
The Chronicle of Philanthropy
Greater Philadelphia Cultural Alliance
LaSalle Nonprofit Center – Phila
Philanthropy Network Greater Phila
Opportunity Knocks
Urban Affairs Coalition – Phila

These regional consulting firms undertake searches for key nonprofit leadership positions:
Bryer Consulting Pamela Bryer, MPH, CFRE
The Burd Group Nancy Burd 215-694-3724 
Diversified Search
Rosewood Consulting Group Priscilla Rosenwald (610) 420-7661

Greater Philadelphia Senior Executives Group – Nonprofit Subgroup
GPSEG is a professional association of select senior-level executives committed to the exchange of business contacts and ideas in a spirit of generous giving, to foster career fulfillment, and professional and personal growth.

Monarch Staffing – job placement in Delaware, Chester, and Montgomery County PA.