Well I have a standard power socket attached to the bike, just one of the cheap ones from Maplins with about 3ft of cable to easy move about. Then I got a 12v-5v usb converter from ebay for a few pence. I'll find some links.
Car Accessory Socket - can't find the pre-cabled one I just put some eyes on the end and attached it to the battery.
USB car charger - just plug in where you need it. Has the advantage of being usable in any vehicle.
I'm sure you local Maplins will have all you need.
If you want it switched then I tend to pull the power from the yellow supply (high beam) in the fuse box. I haven't bothered as the socket is under the pillion seat so is always live when needed. It also means when I charge the battery I just plug it into there.