it's not hard to fill the gaps with brass. It isn't absolutely necessary to cut slots for the tabbed mounts. I make my own out of bar stock if I'm making a rack with stainless.

Nova sells the llewellyn socketed rack parts https://www.cycle-frames.com/bicycle...ess-Steel.html

I can't find the Paragon Machine works rack tabs, maybe they are gone. They were plugs

Compass also sells rack tabs