These are only sold as an add-on item to a pi-Stomp Core kit for your convenience.

If you don't like unplugging a power supply or using a switched power strip, add an inline switch between the power supply and the pi-Stomp.

5.5mm x 2.5mm barrel.

Note that these are for the somewhat unique power plug used by pi-Stomp.  It will not work with the 5.5 x 2.2mm plug common for 9v effects power.