Hi!!
Others can probably give better answer than me. But the ABBC usually reads manga. For manga, I buy on bookwalker, and then take screenshots before running mokuro on that volume. That way I can open the HTML page in a web browser and use Yomitan!
Unfortunately for bookwalker, it is not easy / doable to easily transfer them to other media.
When I buy books, I buy from rakuten / kobo. That way I can get them in calibre as an e-pub, use a tool to remove the DRM and then I can read it on ttsu and use Yomitan.
For shipping books I have used and know others use this site: https://www.cdjapan.co.jp/ however, the shipping is still expensive, so I would recommend to buy lots of books when you first buy.
I recommend searching the forums on things like bookwalker, kobo, rakuten, mokuro and you might find a solution that best fits you! And maybe someone else who can answer your question better 