Curious why you would not want to connect them to the existing wiring - unless you are concerned about total load on those wires. I would suggest now might be be good time, if you haven't already done so, convert your headlight(s) to LED, and then, adding the new lights would be well within the electrical capacity of the system. A big benefit of using the existing wiring, is you won't have to worry about additional controls.
Alternatively, you could tap into existing wiring using standard relays. This would meet the requirement of drawing main power through dedicated wiring/fusing, while still retaining the benefit of no change to switch operation. I'm pretty sure I could offer you a wiring diagram, were you interested.