If you have one and a printer, could be a good time to learn some CAD software.
Aside from for-parts units, that's basically the only way to get old test equipment plastics these days. I've seen the part and probably have the right skills/equipment to make a good version of it, but I no longer have it - was on an SMIQ I got rid of some time ago. Even if you're not great at making the model, with a set of calipers to make measurements and then a few iterations of test prints to try fitting on the instrument, most shapes are not too bad to get close enough to work, though there are some geometries that are just hard to print because of the way 3d printing works.
Otherwise, scour ye olde ebay for busted units or parts sold as spares and be prepared to wait.