It would be helpful if we were using correct (common) terminology. Define "spacer". Are you looking for a spool? (part between disc blades) If so, 3" is a very odd size. Is that a half spool? By "center bar", are you referring to the axle?
Agri-Supply still list's quite a few "generic" disc parts in their catalog. Also, Valu-Bilt has many listed in their catalog.