Got mine at NAPA... two kinds: regular and marine.
I assume the marine one is water proof to some point.
Went with the regular one and mounted it in the cab to select
between either, both or no battery position.
pre-ran with it, so far so good.
Use number one cables, boot it as you go through any firewall or floor-boards.
I believe you are looking for a Cole Heree, Hella, or Perko style, they kind of look like a claymore mine. Be careful where you mount them as we have found that the Cole Heree unit, over some time, will lose the detent / locking positions if the back of the switch is placed in an open exposed area.
Take a look at the Blue Sea Switches. They are small and compact yet pack a great punch for value and can be flush or surface mounted.
Please feel free to call or e-mail me with any questions.