If nothing else works, drill a small, easily filled hole in the drywall near where you want to mount the shelf supports. Bend a wire hanger (remember those?) to a 90° angle and thread it through the hole.

When you rotate the hanger it will hit the stud and you should be able to tell where the stud is with enough precision to attach a shelf mount to it. Or maybe you'll get lucky with the 1st hole...