Sheaves are measured by diameter and width to handle a certain size line: 5/16, 3/8, 1/2, etc. You can probably source them in numerous locations; however, rig-rite / Zspar might have them, as they have the molds for the tabernacle and step plate. On the other hand, they have had those two parts on backorder for the 272 for something like 7 years if I recall a recent conversation with them correctly, so I don't know if they would have any of the other parts on hand.
Thanks,
Andrew