What type of connector do I want DC Plug 2.1mm/5.5mm or DC Plug 2.5mm/5.5mm?
I have a Diecimila if that helps. Also I already looked in the getting started PDF and the hardware page and still can't find the answer.
I'm about to buy one from Maplins: Search
Update: Found the answer (wasn't looking hard enough ;D) Arduino Playground - WhatAdapter