Aren't all music stores DRM free these days? I personally use Amazon, but iTunes or... hmm, you're right, a lot of the big name digital music stores have now shut down.

Bandcamp is the pinnacle of such places in my opinion. I’m hoping it stays as it is for many years to come.

Qobuz sells drm-free standard and hi-res flac files.

iTunes still has a store where most music is DRM free, and available in a lot of countries (as opposed to Amazon MP3, which is only available in a few). If you have a Mac it's fairly easy to right-click a purchased, downloaded song in Music.app and click "open in finder", so you can copy it (or the entire album) to some self-hosted music server/app.

There are sites like bandcamp, boomkat, beatport, bleep, 7digital, but they all have smaller collections, so may not have what you're looking for.

There's also budget options of questionable legality hosted in countries with a more flexible approach to music licensing, like e.g. mp3va.com.

Amazon has MP3 downloads, at least where I live. Last time I bought a CD there, the corresponding MP3 download was included.

Bandcamp. A lot of artists just sell their stuff directly on their Bandcamp page now, and you get the choice of format when you download (MP3/WAV/FLAC/etc).