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What many people do is buy them from places like Kobo.com, download them, and remove the DRM (usually via plugins to the Calibre library management software).

Kindle books have become impossible to download, but other online bookstores aren't as restrictive.

https://standardebooks.org/

If you want to buy them legitimately, there aren't a lot of options

Illegitimately, there are tons of options

https://www.globalgreyebooks.com/index.html

Shelfmark (+ Grimmory/Calibre) is magic.

Anna’s archive