I believe it is the 2.5mm internal diameter. This not as common as the 2.1mm cctv ones. You need to buy what they call as 5.5x2.5mm pigtails.

I found a 12V30A power supply and the US power cable for it to go to the wall.

I've been looking for 5.5x2.5mm pigtails but only seem to be finding a ton of 2.1mm (like you said, much more common).

I'm trying to find a source. I'm based in the US.

Are those octocables easier to find or something?

http://www.ebay.com/itm/Straight-DC-5-5x2-5mm-male-power-plug-cord-connector-cable-for-adapters-2pcs-/131106289737?pt=US_Surveillance_Cables_Adapters_Connectors&hash=item1e868b3849

This looks like one but it ships out of China and will take way too long to get here.