how can i find a replacement knob for my very old magnalite cassarole pot
It depends on how it is attached. Then look for any brand any source ovenproof knob that fits that method. It needn't even be for cookware (example: Ceramic cabinet knob). My crock pot cover's handle is now a drawer pull held by a stainless steel screw. Bring your cover to your full service local hardware/home store.