How do you get a job as a stand in?

Stand-ins are taken from the same places they get extras (like Central Casting). But a lot of people like to do this type of steady work, and there are not many slots, so they are hard to get.

And you need to be the same height/color as the person you are standing in for.

Also if you have a calling service, that could help if you specify that you are interested in stand in work.

As Iceman said, it really depends on your sizes, and physical resemblance to the actor/actress you would be standing in for.