DXE sells the rotor platform but it was out of stock when I was looking for them.
I bought the same pair direct from a company in Germany for fifty bucks cheaper with shipping. I also bought the thrust bearings from them which were priced good and to be sure they matched up with the platform holes.
Heavy platform for rotors or upper bearings, up to 60mm
www.wimo.com
The bearing holes are supposed to match up with Yaesu thrust bearings and rotors. My rotor and bearings were a perfect fit.
I do strongly recommend you use 2 rotor platform plates and thrust bearing if using a heavy antenna on a pole like I'm doing. I believe you can see why that is.