Perhaps you have to buy the whole rim and center plate to get one. While looking for a replacement, why not just weld up the crack. Drill the crack at the end to kept it from running unless it is all the way to the edge already, V bevel it about 3/4 of the thickess and weld it up with 7018 weld rod. Flip it over and to the same then on the other side going into the previous weld a bit to remove any slag (about 1/2 of the thickness this time. With proper welding it should be stronger than the original steel.
* After drilling the end while cutting the V in with a grinder, go past the drilled hole by at least 1/2" and then weld to the end of the grooves from both sides.