Now that #amazon has ended downloading offline copies of their ebooks, I buy DRM-free EPUBs from ebooks.com. Not all books are DRM-free, but they have a dedicated section.
https://www.ebooks.com/en-us/drm-free/
Examples:
The Wandering Earth (2017) by Cixin Liu
https://www.ebooks.com/en-us/book/210185098/the-wandering-earth/cixin-liu/
All Systems Red (2017) by Martha Wells
https://www.ebooks.com/en-us/book/95651192/all-systems-red/martha-wells/
Mistborn: The Final Empire (2006) by Brandon Sanderson
https://www.ebooks.com/en-us/book/630871/mistborn-trilogy/brandon-sanderson/
The Terraformers (2023) by Annalee Newitz
https://www.ebooks.com/en-us/book/210542637/the-terraformers/annalee-newitz/