Andover Norton, RGM or Mick Hemmings should supply the hardened centre. Beware of cheap suppliers as its cheaper not to harden them so what looks like a bargain price might be a cheaper none harden part.
Mine came from RGM (as part of belt drive kit but is sold separately) the blue tempering lines of the heat treatment process can clearly be seen on the splines that engage with the clutch plates.