I’m looking to change my reading and buying habits away from Amazon and need an alternate source for my ebooks. Where can I buy ebooks that won’t expire or have their licence revoked and will allow me to download a copy of that ebook I can store on my own system regardless of the websites status?
This is unfortunately the closest answer to what the OP is asking.
To the best of my knowledge, outside of independent authors like Cory Doctorow selling drm-free from their own sites, I’m not aware of any place to shop for ebooks that are not tied to some online DRM scheme.
Humble Bundle books are another option, but it’s highly dependent upon if you want what they’re selling.
Baens library has been doing this for the past 25(?) years. They even have a free library. https://www.baen.com/catalog/category/view/s/free-library/id/2012