Getting harder to find these days since most people rely on their phones now. Best thing for iTunes is still the Nano or iPod Touch in my view, although 7th gen Nano or 7th gen Touch are not cheap. You can get an iPhone SE for almost the same money. The Touch was phased out last May 2022 but can still be found. The problem will be support from Apple. My wife still has a Nano and I turned off automatic updates on iTunes so that it doesn't brick her player.
I don't think any of them have expandable storage though. Sony still makes a digital Walkman. The cheaper ones don't have much memory but some models do have SD card slots. Onkyo makes an mp3 player which has an SD card slot and reviewed well. A number of sites mention Astell & Kern but those are higher end players. A good starting point might be
this article on TechRadar. Hopefully some other members can recommend something as well.