Mine would break by looking at them hard. The service centers always replaced them. Back in the day they had a supply on hand because they broke a lot. Last time they had to order them and took a couple weeks. I'd check with them. Of course, if it's still the same part, it'll still break easy. The only reason I went few as few pairs as I did was because I very rarely turned on the fan or changed the balance knob.
If you search the roadster forum, I recall an old thread that had a pic of different replacement knobs the owner had found on their own and put on. As I recall they were sting metal vs. plastic and looked like they wouldn't break.
I remember reading somewhere where someone was replacing them with nice shiny metal knobs which are more robust.
There is a picture I saw online i.e.
Yep, that's Tesla 55, as I mentioned in post #3 :smile: