Are there still any websites where you can buy and download Japanese ebooks to a PC?

Hi everyone, I’ve recently been trying to buy some Japanese ebooks, but I’ve realized it’s becoming nearly impossible to actually download the files to a PC from major stores like Amazon or Rakuten Kobo. It seems Rakuten Kobo has also implemented restrictions preventing users residing in Japan from downloading their purchased ebooks directly to a computer. Since many of the titles I’m looking for are region-locked and not available for purchase on international stores, I’m stuck. My goal is simply to get the files onto my PC so I can use them with reader.ttsu.app. I own a Kobo e-reader, but I find the reading experience there incredibly frustrating compared to the efficiency of using a pop-up dictionary on my browser. Does anyone know of other Japanese ebook stores that still allow you to download the actual files to your computer? (And just to clarify: the books I’m looking for aren't available through "alternative" sources, so buying them is my only option…) Thanks in advance for any help!