source for microscope parts?

[Allison]

We have a Leitz Diavert microscope that is in good working condition 
except for the fact that it is missing a phase ring.  Unfortunately, the 
microscope was bought in the 80's and is obsolete as far as Leica is 
concerned....so the part number for the phase ring is no longer in their 
system.  Does anyone know of a good source to get parts for an old 
microscope?
TIA
Allison