I'm looking to buy an MP3 player. My reason for not using my phone as an MP3 player is that there simply aren't any good MP3 player apps on the iOS App Store. Everything either a streaming app, or "free with ads".
Criteria isn't strict, but no devices that are just an Android with the cellular modem taken out, which I see a lot of listings for. Rockbox support is preferred but I've had no luck finding anything in their supported devices list that's not an iPod, used or new. I'll probably get a used iPod if nobody has an alternative recommendation.
iOS has KMPlayer which was good the last time I used it: https://apps.apple.com/us/app/kmplayer/id835776444
Also Plexamp if you want to use Plex.
Walmart has a big selection of mp3 players: https://www.walmart.com/search?q=mp3%20player&typeahead=mp3%... However, i was not able to find good enough one that I would recommend. You can try searching yourself.
I personally use Plexamp and Musi.