Bert, when you get into Laredo, just head toward Mexico.
You can't miss it.
It's right off the Interstate - I35, I think.
From Billie's explanation when she posted the Sonora Flag on The Finest:
"I took this photo near Sonora, Texas, on I-10, where the interstate cut through this mountain. The flag, flying so high above us, was just overwhelmingly beautiful!"