I don't know if you'll really find a "source'. SignComp and others sell kits for directional 
signs, but these are an easy manufacture. They look like your run-of-the-mill directional 
sign fabricated from 2" square aluminum tube frame/supports and overlaid with .063" or .080" aluminum. Your dimensions indicate they're 5" deep. They don't look that deep (again, I would assume 2"), but if they are 5", it's still an easy job if you have basic tools. If you don't have the means to manufacture them, contact Kevin Crutchfield at Casco 
Signs. He's pretty close to you (in Concord).