Are you sure 9/16 is too small? Crank bolts are usually 14mm, not 15.
Many older bikes have 15mm crank bolts; occasionally the standard 15mm sockets are too thick to fit inside the cut out portion of the crank where the bolt is.
as far as a replacement for a 15mm wrench, 15mm is the standard size for axle nuts and also pedal wrenches (I don't know how a pedal wrench would work, 15mm crank wrenches OTOH are often used as axle wrenches and you may have one.