ugh, how to repair a pepper grinder when a part has gone missing?

bought a cheap acrylic one on sale and the knob felt gritty from day one.

took the top off to clean it and now the little nut that holds the burr is somewhere on my kitchen floor. every thread i found on how to repair a pepper grinder assumes you still have all the parts.

can you even buy that nut seperately or is it bin time?