Worth investigating: NEW Walabot DIY Stud Finder In-Wall Imager.

Also consider *avoiding* the metal studs and using one of the Wingits fasteners. These are considered acceptable for securing grab bars in drywall. Drywall is very strong if the pressure of a fastener can be spread out. Wingits do that, but I am not sure they are rated for the high static load.

At least one type of Wingits (RC-MAW35-2) requires a 3/4" dia hole in the drywall, and claim to work for walls up to 1.3" thick. Such large holes might be ruled to compromise the fire rating.