I was going to suggest Charles Hyde & Sons here in the UK, but a quick search of their parts database indeed shows the 29122WY as Discontinued. They do however still list part 29110B which may be similar enough to either do the same task or be used as donor parts to fix your existing fader(s).
Often these faders are just special order from Alps or Panasonic, where they may have longer shafts/cases/pcb pins, so you may be able to buy very similar parts as standard order items, then use them to replace the parts of the faders that are faulty (the enclosures often have bendable metal tabs to enable them to be taken apart fairly easily). I've done this on more than a few occasions.