[Tally]

I mean, a bit more searching did seem to turn up a few notch beamsplitters near the required frequency (not quite the same frequency though)

I am wondering if part of the reason that finding these at the frequency of sodium vapour lamps is partly due to those lights no longer being common. Or perhaps that those just aren't frequencies common to laser optics, which seems like the biggest market for these kinds of devices anyway.

Though, I see no reason why you couldn't use a laser for this process anyway, with matching notch filter.

[delfinom]

589mm notch filters are readily available

https://www.edmundoptics.com/p/589nm-cwl-125mm-dia-hard-coat...