If you can’t find where Rampage sources the LED from, then you’re probably stuck buying the whole assembly. I don’t keep up with Rampage much but I can say I have never seen just that LED for sale. Always the whole thing.
The YJ forum isn’t chock full of good information on the third brake light anywho, as only 2 years of the model had a light. And a lot of those are long removed by now. Not trying to be a downer, just don’t expect a lot of information on this subject here, especially for something so specific as one brand. I am putting a new TJ assembly on mine, I think they’re a good idea to have.