I dont think so u get spare parts for such stuff. Its like either u go with the entire set or search for some alternative look alikes which wud do ur job...Take it to the luthier or sum spare parts shop..
Take a print out ofthat knob pic and ask for alternatives in electric, metal, Hardware all the spare parts shops that u can think off..I am sure u will find sumthing or the other which wud do the job...
Guitars : USA Fender Stratocaster 1981 Collectors Series, Ibanez RG420eg, Cort X6vpr Snake skin, Custom Tele Style Guitar
Processor : Boss ME 70
Amps: Laney Cub10(4 Sale), Roland Microcube
Recording : Focusrite Scarlett 2i2, Shure SM57
For Sale: Sansamp Tri-AC