Okay tried doing some research on this, but does anyone have tips to finding the junk yard dog? I can find the lady who sells cows all the time but not the dude who sells the dog.

It's random I think.

This is one of the things that I am waiting for the GECK to fix. A mod that shows the location of every random encounter in the game.

He randomly spawns in some certain locations when you visit them, like enemies. There are spawining spots on the map. I met him 5 times, 4 of them happened near Starlight drive-in. When you fastravell there don`t go to the settlement area, turn around and go to the red train bridge. There is a random encounter spot under the bridge. You can meet him there . Though it`s random, so you also can meet there some random enemies instead or find some corpses for looting.