This is the store I ordered it from, directly from the manufacturer (Yajiamei):
https://www.aliexpress.com/store/219000/
It was sold as a “XP-E/XP-G lens 17mm diameter conical stent” or something like it. Sadly the shop seems to be gone - maybe just not selling to EU anymore, but for me it is completely empty.
I have found this shop which seems to belong to the same company, and which still sells the lens model I bought:
You need the lens with conical holder, then carefully slice the top off the holder. Watch our while doing it, I tried with a knife at first, slipped and paid the blood tax 
I used 15° bead surface, which gives a well balanced beam, but they have others. Sadly this shop seems to sell in lots of 10 units, so the price if you wanna order 3-4 different lenses is pretty steep… The old shop sold single pieces for ~60ct 
For reference: Yajiamei is the company that seems to make most of Convoys TIR lenses. Their quality is good.