My car has a 12V power socket in the boot, attached to the parcel shelf support on the right hand side. I took the supply from the wire that is connected to the central electrode of that socket, which is live when the ignition switch is turned to the 1st position.
It is a 2015 face-lifted SE, I don't know if the socket was standard on lower spec versions, but the wiring might be there, even if the plug connector is just dangling loose.
On my previous 2011 i30, which had no socket, I had to run a new wire to the fuse board by the drivers' right knee, which meant pulling the trim off along the bottom edge of the doors so I could push the wire under the carpet. On my car, there are some spare ways in the fuse-board, though I've never had a look on the back side of it to see how to connect wiring to it. Alternatively, you can get a piggy-back fuse adaptor that plugs in like a normal fuse, but has a flying lead connected that you can can connect to your new supply wire with an insulated butt splice crimp, or even a soldered joint with some heatshrink tubing over the top.
