I don't know if you can get one ready made, but I ordered one like this: http://compare.ebay.com/like/230593...ffb89b3&itemid=230593989063&ff4=263602_325002
and had the center opened up until it would fit way onto the back on the pulley and then welded. They spun it on the lathe after to make sure it was true.
I have one year on it so far without issue.
-Jon