Or if you want something selfhosted, set up Calibre. You can just upload the book to your Calibre library and one-click send to kindle from there.
If you need to find any books, check out Annas Archive at https://annas-archive.org/ . Download any book you want, then you can just email it to your Kindle.
Discussion
I wish the DeDRM plugin still worked 😕 it was the only way I could justify purchasing ebooks.