As BP said, the conversion kit for old Zipp discs is incredibly difficult to find. If you want to go old Zipp 900 route, I'd suggest sourcing the conversion kit first, then find the wheel. Or, you can look at an older Zipp or Mavic track disc. Lots of people still run those. Just make sure they aren't dinged. I've been told that the aerodynamic differences between the best and worst rear discs isn't that big.