Amazon.com: DZS Elec 6pcs 15CM 9V Battery Snap Connector Clip Lead Wire Experimental Battery Snap Power Cable With 5.5x2.1mm DC Male Plug for Arduino Project or LED Light Bar : Electronics
Adafruit 9V Battery Case w/ Switch and Barrel Connector - RobotShop
$1.39 - Heavy Duty 9V Battery Plastic Connector - Tinkersphere
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