yes as far as i know gm does sell just the button, may be wrong, someone will correct me if i am. if not you can use another button. but they can be hooked up the way you want.
i dont know about attaching them to the square brush guard, maybe custom brackets, or someone already sells square ones.
i dont have lights like that so i cant recommend any but i have always liked the look of PIAA 520 and some of the others.
http://www.piaa.com/Lamps/Lamps-General.html