Where to legally bulk purchase MP3 in 2022?

If I'm understanding your question correctly, I think you're looking for something like Bandcamp which lets you pay to add music or other audio items to your collection which you can stream or download in high quality formats

https://bandcamp.com/

Bandcamp (https://bandcamp.com) provides DRM-free MP3 downloads, as does Amazon Digital Music (https://www.amazon.com/MP3-Music-Download/).

I download them from YouTube. I don’t think that’s illegal (The YouTube video has to be downloaded to my computer in order for my browser to play it. I just go one step ahead and extract the audio as mp3)

Most music services that sell music sell it DRM-free, so even if it's not MP3 to begin with, you can buy it, download it, and then convert it to MP3 or any other format with something like ffmpeg.

Also, the vast majority of physical music CDs have no DRM and can be ripped to whatever format you want. This is more inconvenient but can be much cheaper than buying digitally especially if you are looking for popular music.

For the longest time I just got all my music off of the archive.org live music archive.

Not a super huge selection but I could always find something interesting to listen to. Also found a bunch of bands I would have never otherwise from “so and so from $band_i_like did a set with $other_band” and checking out some shows from $other_band.

Quobuz also sells 24-bit audio files. You can do this without a subscription, but certain subscription tiers provide a discount.