Amazon.com: 5pack 9v Battery Clip with 2.1mm X 5.5mm Male DC Plug for Arduino by Corpco : Electronics
5pcs/lot Experimental 9v Battery Snap Power Cable To Dc 9v Clip Male Line Battery Adapter For Arduino Uno R3 Diy Jack Connector - Integrated Circuits - AliExpress
Switching voltage on/off from a 9v battery using a 2n2222A transistor and an arduino - Electrical Engineering Stack Exchange