If you are talking about the genuine "big rig" style of mirror, all I ever saw came with the brackets.
There are brackets that bolt through the door-skin, then the mirror supporting rods, and whatever mirror head you chose to install.
I never knew them to be truck specific.
A couple little pointers:
Buy nothing except STAINLESS STEEL.
Anything else will rust, and then big streaks of rust will wash down from them, across your shiny paint.
This part is VERY important...Mount the brackets as far FORWARD as the door will allow.
For some stupid reason, most factory mounted truck mirrors will be mounted way too far back toward the center of the door.
This forces you to lean back and reposition yourself, every time you look behind you.
With the mirrors mounted far ahead, all you have to do is shift your eyes to see behind.