I purchased these keycaps as well. They are from PFU.
https://smile.amazon.com/gp/product/B00FPGOZTQ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
They also have the Color keys as well, if you are so inclined.
https://smile.amazon.com/gp/product/B00HC2W0FC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
It’s the official kit from PFU, that come with the extraction tool and 4 keycaps (escape and control with and without print). I found it on amazon, was the fastest from France. Here is the link on American amazon : https://www.amazon.com/PFU-Color-HHKB-Professional-PD-KB400KT01/dp/B00HC2W0FC/
The next keycaps I ordered are from eBay, but will take more time to arrive. French amazon have less choice. They are nice web site selling keycaps but can be a nightmare to ship to France.
About keyboard usages, I started using the HHKB Lite at end of 90’s, the Pro 2 since some years, and my Pro Hybrid is my main keyboard now ... it’s a pleasure to use it with my iPad Pro, so I take it with me everywhere I go 😇
I've found some OEM keycaps on Amazon recently:
Green or blue PFU R2 modifer sets
Latter set shipped fast, at least for me. Arrived ~1 week.