they are difficult to find on PMW the site because they make sliders, dropouts, and inserts.
I've owned two frames with inserts like that- Vassago and Soma. (I think US-made Vassago frames like the Verhauen used genuine PMW sliders/inserts, but the Jabberwocky was a bit cheaper by using generic sliders and inserts.) I think both used IRD sliders, which are basically the same thing as the PMW sliders. they are not a perfect match, but they should work interchangeably. PMW will tell you they are not the same thing, but that's mostly because they want you to buy their product, but also because they are not exactly the same thing, even if they work together.