It's just a dichroic beamsplitter, which is a standard optical element available from online sellers. Variants of these are commonly sued in scientific experiments, optical devices, etc...
Here's one for under $300: https://www.meetoptics.com/beamsplitters/plate/longpass-dich...
Just search for: "589 nm beamsplitter".
However, I'd guess these are the components the guys are using between the prism and the cameras.
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.
https://www.edmundoptics.com/p/589nm-cwl-125mm-dia-hard-coat...