[Barnabas]

It might be cheaper to buy some gumdrops from a candy store and mash them on the pots.

Seriously, if they don't have to look like the originals, the hole in the knob that mates to the potentiometer shaft is fairly standard. You might find some knobs from an old piece of electronics that might work. Check with a local equipment repair company. They might have a box of old knobs they might let you look through. Knobs from old mixing consoles, or even the volume control from old TVs might work.