Servo arms (horns) for potentiometer
They don't exist. You're asking for an accessory made for a device (RC servo) made for one market that will fit onto a device (potentiometer) made for a completely different market. You will have to custom something yourself.

You could buy a piece of rectangular stock of plastic or metal,
drilling or reaming (if plastic) a hole into it for your pot shaft and drilling and tapping
a hole for a set screw. Unless you are able to machine a D-hole, in
which case you wouldn't be asking this question in the first place.
You could also buy a real, high quality, aluminum  servo horn with a thick hub and and drill/ream out the hole for the spline since
the average servo spline diameter is 3mm and your pot shaft is 6mm
which is much bigger. And if the hub has enough thickness you can
drill and tap a hole for a set screw.
Buy the plainest cylindrical (i.e. flat faced, straight sided, cylindrical) knob you can find that will fit your pot shaft and drill and tap some holes into and bolt a lever to it.

You'll need a drill press for the kind of holes that need to be drilled in all this.