can just convert a grand wave sealine handle for your sl30.. https://www.youtube.com/watch?v=fIc1qHwd4mc
fin nor handle fits a 15/30 shimano
have a look at some tackle shops which handles they sell separate and see if it will fit your reels
if you looking only for the knob itself its not easily available and you might have to buy it from the states from sites like reel power handles..also comes fitted with handles if you don't want to drill holes to remove original knob and pop rivet new one on handle etc
Within the same brand, you normally have some compatibilty - here is a pic, of one of my sl20sh Daiwas, with the factory handle of the GWX/SHV fitted - the BG series is even nicer !