Source for dichroic filters

I am looking for a 25mmx25mm dichroic filter for a DeVere 504 lamphouse. I want to replace the cyan filer with a neutral density filter, to use the lamphouse for B&W VC printing.

That is an interesting way to do it. Frequently RED is used for NEUTRAL DENSITY when working with orthochromatic material (like conventional multigrade printing paper). RED is easy to conjure up as a mixture of MAGENTA and YELLOW.
Since you don't want to use RED, I'd not bother with a dichroic ND filter. What i'd do is use a piece of screen or opaque piece of metal. That is how most color enlargers implement intensity control without causing diffraction or altering the light spectrum.

For example these are the neutral density filters for one of my sensitometers. They are not dichroic. They are opaque with clear areas. They don't fade or change the color of the light.

On the Durst, the Density (Light Intensity) is an opaque metal shade parallel with the color filters. It is not a neutral density filter.