For one trailer, just buy the the darn things ready made:
www.noertherntool.com Item# 125504

Michael Shaffer wrote:

I'm trying to add sides to my trailer but it doesn't have the metal
brackets to put the 2x4's in. I want to make some and weld them on but I
don't know where to get the metal. I don't want to have to buy a whole
sheet, I just need a 3" wide strip I can cut, bend and weld. Any ideas?

Thanks