Internal parts are not available to buy.

You have to search for damaged / used keyboard so you can pull the part from it… or search for similar rotary encoder (6-pin) / potentiometer and solder with a new one.

Since i have a similar Problem with the volume knob and other might too:
It seems like a P140KH-F15????
https://www.ttelectronics.com/Products/Passive-Components/Potentiometers/P140KH

I measure 5k ohms, but be sure to verify that, no guarantees.
It also seems like an audio taper, but it might as well be reverse, since i did not check polarity.
I would assume therefore:
P140KH-F15AR5K (audio taper)
P140KH-F15CR5K (reverse audio taper)

You find such parts for 1-2$/€ at Digi-Key, Mouser or any other electronics store. But be aware the is always a minimum order value (50 $/€) or so.

No guarantees for correctness