I'm thinking about fitting a rear spot light like this one:

first question is where can i get one like this or do i just buy a pair of spot lamps, and does it need to be a special beam pattern?
secondly, How would i wire it into the existing reverse light circuit while still keeping the 2 origional lights in use and would it be legal to have this fitted allong with the 2 reverseing lights i already have on my car (1988 Designer)
lastly, would it be legal to have it wired up on its own circuit so that i could turn it on at any time, i know it wouldnt be legal to have it on while driving forwards, but i would quite like to have it wired up so i can use it as a spot light when im camping etc.
Thanks
Chris