I've found most Amazon downloads to be 320k and, while the audiophile will argue, hard to distinguish from FLAC. Most importantly, it's higher quality than the EP I have on magnetic tape that was a recording of a recording I bought at the Fireside Bowl in 1999. Boy, I enjoyed listening to that, and the 320k MP3s do just fine so long as I don't need to re-encode them anytime soon.

Bandcamp is the standard go to — if they’re on there you can get lossless quality. Alternately I believe Apple Music is now selling the highest quality digital downloads of all the major providers at the moment.

7digital has a decent FLAC catalog.

Nonaudiophile here.

For classical music and jazz, I have found Presto Music to be a good place to buy mp3s. The search function works well, and many classical albums come with liner notes in PDF booklets.

Presto also offers FLAC downloads, but I can't hear the difference on the equipment I use so I buy the cheaper mp3s.

I have also bought mp3s from Amazon. I didn't have any problems with the sound quality.

Qobuz sells lossless FLAC files. I use their streaming service and will then buy albums if they are something I want to preserve. They also have excellent human curation.

They work for me, but I don't consider myself an audiophile per se.