Where does one purchase POM keycaps?
As Skull_Angel mentioned, the black Cherry G80 and G81 boards have POM caps (although I'm not sure about the stabilised keys, on the light grey boards with PBT caps the stabilised keys are ABS).
PLU ML-87 and ML-G3000 also have POM caps, as does the black Keycool 108. You can sometimes get blank black POM caps from Banggood (87 key and 84 key sets).
The rainbow jelly POM sets on Taobao can often be purchased with a few different options, one of which is a full set of white jelly POMs.
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Why POM?
It's the densest keycap material so it adds more heft to the keys and feels different to both ABS and PBT. It doesn't get "sticky" like smooth ABS or "dry" like PBT, but remains "slick" a little like Teflon, but not greasy. POM keycaps tend to dampen the higher frequencies that key switches make, so they have less "tick" and "click" and more "thock" and "thump". They just feel like quality, especially on a solid plate mounted board.