The back of the light switch is a good source for an ignition live, its the black/yellow wire.
The safest way to wire it up would be using a relay and the ignition live wire to trigger it, power the relay from the battery with an inline fuse.
Having said that most led's are very low current so you could probably get away with wiring them direct to the back of the switch, with a fuse of course, as long as you don't use the high power led's that you can get. The stick on strip type are likely to be very low current.
There is a grommet in the corner of either end of the area under the windscreen wipers, remove the wipers and the plastic panel just pulls off to give you access to them. Both come through into the cockpit and are easy to get to to pull your cable through.