I think it was originally from IMSTO, one of his group buys perhaps. I got it from another member. There's also an HHKB layout plate available from IMSTO too. Not sure if he still offers those for sale. You can try GON though for HHKB layout, his plates are very nice quality.
Here's the HHKB layout plate from IMSTO (no soldering, I was just testing stuff)
In other news, I've swapped to 62g gold MX Blacks already--MX Whites were an interesting experiment, but felt wrong on that board. Super smooth MX Blacks feel just right! (typing on it now). Also I've learned that housings matter for clicky switch stems: vintage/old MX Black housings made the click much softer and more inconsistent, but new MX Blue housings I tried as a test had all the MX White stems I've tested perfectly clicky. Maybe something to do with the shape of the metal leaf, which I always am very careful to not touch. Also no lubricant used in the bottom housings originally. I know I've used vintage MX Black bottoms for Clears with no issues, but apparently clicky switch types had slightly different leaf shape? Just a guess so far, based on this experience.