While I'm almost certain that it's a common (and cheap) off-the-shelf part, linear potentiometers can be a
bit difficult to source just because there are so many styles and values to choose from. Have you called
ETC? They are very helpful with these kinds of requests and probably have the part in
stock. If you can locate the part number or suitable substitute, Mouser or Digi-Key will probably be your best bet.