I'm with Barb, as scary as it seems to design your own quilt, this would be a really easy one to design - grab some graph paper and sketch out a general idea of what you want, then find pictures of trucks and such anywere - coloring books (as mentioned), online, your sons favorite books... and blow up the images to fit your general pattern! Trace 'em, cut 'em out, and start going.
It's a really really cute Pottery Barn set. If I wasn't so knee deep in the dinosaur quilt (and my son was all into cars instead of dinosaurs), I'd be making something very similar. Good luck!