9V battery clip with 5.5mm/2.1mm plug : ID 80 : $2.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Amazon.com: 9 V Battery Clip Connector, I-Type 9 Volt Battery Clip, 9V Battery Buckle Connector Hard Buckle Plastic Housing and Black Red Wires Leads Iength 5.9 inches (I Type Hard Shell 8PCS) :