A 2.5mm barrel jack is really not common. It is used on some special board like ODroid-C2 and other device manufacturer that prefer unusual barrel jack size to have a better control on their devices.
With this cable, you will be able to use/wire your own power supply to a 2.5mm barrel jack socket.
- Outer diameter: 2.5mm
- Inner diameter: 0.8mm
- Center positive (the red wire)
- Outer negative (the black wire)
- Cable length: ~270mm approx.