You can probably order any wiring kit for any aux lighting system you want. Just about any of them will come with some wiring, a couple fuses, a switch and a relay.
That's basically all the parts you'll need so if you want to just buy the components separately you can. This way you get the exact kind of switches and what not that you want.
Anyway, I would use a "fuse tap" sometimes called an "ADD A CIRCUIT"
fuse tap - Bing Images
Find a fuse that only has 12v when your ignition is "ON". use the fuse tap here. you need to use two fuses, one for the existing circuit, and one for the addition. Run this wire to one side of your SPST switch, From the other pole go to pole "1" on your relay. go from pole "2" on your relay to ground.
Now, run heavier gauge wire from your pos battery terminal through a ~20a inline fuse to the "3" pole on the relay, then run heavier gauge from "5" pole on the relay to the pos wire on the lights. Now just land the lights ground wires to ground.
If you have a backlit switch, it probably has three more spades on it. You need another fuse tap in a circuit that's powered with the running lights. Run this to the spade for the switch light ("6" on the image below) then ground the switch's ground spades ("7" "8" on image below).
http://www.4x4earth.com.au/forum/images/posts/wired.gif