You may be able to buy the parts from Digikey.com or Mouser.com.

There usually is a number on the side. Most are made by "ALPS".

You need to match the resistance value and the "taper" which is indicated by a letter following the nunber. You will want an "Audio Taper" for these functions.

Replace ANY flakey ones because there are a LOT of screws to remove to get at these.

You sometimes have to cut the handle down to match the original ones. Use tin snips to cut and file off any burrs.