Gene,
I don't have a "favorite," but use any and all sources I can find. Just recently traveling in Maine I turned and caught a sign that said "Low Clearance, 12' 8"" Well I quickly backed up and went the way I was supposed to. I cannot find that on the AITA web site. I didn't check the MC RA but should have. I'm leary of any overpass that does not have markings on it. I'll pull over and wait for an 18 wheeler to come by and if he clears it, I know I will. 18 wheelers are 13' 6" high.
On Edit: I just ran across this site this morning and may consider it as "another" source.
Rand McNally Trouck Routing
I did a search (Google) on "U.S. Truck Routes."