You need a new plumber. That is the famous ( infamous?) Price Pfister Flowmatic. '70s vintage. Amazingly enough, the Flowmatic Cartridge is still available, but pricey....~~$50 As with ALL makes and models of shower valves, the ONLY handle that fits is THE handle. It is a lever, with a sort of star-shaped plastic piece covering the screw. A good plumbing store will have it, but the hardware stores may or may not, as the demand has fallen way off.
The Flowmatic Cartridge, although still available as OEM brand, is in reality an aftermarket mexico part, and while the originals had a life expectancy of 10 to 20 years, I have seen more like 1 to 3 years on a replacement, so at some point it is worthwhile considering a complete replacement. But get a new plumber!