I had to replace that disc also. I ended up going to a automotive brake specialty shop. This shop rebuilt brake systems, so had excess brake material in stock. Any local auto shop that I asked for bulk brake pad material steered me to this place.
I asked them if they could sell me just a small piece, told them I needed 3/4" diameter disc, maybe 1/4" thick (I think, you might measure again to make sure). The guy went into the back, came back with a 3/4" diameter piece, about 2" long and handed it to me at no cost. I think he probably pulled it out of box of junk pieces.
It was easy to cut to the thickness I needed, using a hacksaw to get to a rough dimension. Rubbing it against a piece of sandpaper laid on a flat surface let me get it to the exact thickness I needed. I still have the rest of the stock he gave me, buried somewhere in the garage.
Finding where to get it took a little searching, but once I had the material it was easy.