Anyone seen this illuminated knob?

I would have to say then Apple would be the best source to start with.

Buying proprietary components is often difficult. Especially with companies like Apple, where full device replacement is often the first step.

However, you may want to look into these:

45mm Multicolor illuminated POT

Illuminated POTS (The 8mm ones may do the trick.)

Or, find the speaker set on Ebay, broken or not, and harvest parts.

Cabe