[Aavso-photometry] Where do the professionals source their V and B filters from?

[Richard Miles]

Further to the troubles people are having with filter glass degradation, it 
occurred to me that some professional observatories carrying out V and B 
photometry may have solved the problem by finding a supply of stable 
filters.  Does anyone know whether this is the case or not?  If yes, then 
where do they source their filters from?

Thanks in advance,
Richard