I bought my Corvette billboard from a guy over on the Corvette Forum. Think I paid about $150 for it. You see them every now and then on Flea Bay.
Mine came in 8 panels. My garage has textured walls so I built some framework of 1 x 3's with a skin of 1/4" plywood. I ran a chalk line to help line up the first row of panels. I took the billboard panels to a sign company. They ran them through a machine that applied a glue, folded each panel, and put them in a large plastic bag. I'm told that wall paper paste will also work.
Putting the panels up is similar to hanging wall paper, except that each panel is a little over 4 foot tall and 5 foot long. The bottom row was easy. The problem is the top row. If everything isn't square, the panels on the top row don't line up properly. Probably not much of a problem when the billboard is going to be 30 feet in the air, but I wanted it to look good from 10 feet.
I'm pretty happy with the final results.