Could just order some from China(1). Have to read the description carefully tho. Usually anything under 15USD not worth buying and over 30USD you're overpaying.
It depends on the version, but you can really trick out iPods. I've got an iPod Video (5.5) with an iFlash Quad [1] (up to 4x micro SD cards) with a 2000mAh battery, running Rockbox [2]
I also replaced the rear panel since I didn't need the beefy 80gb version, and also the lock / headphone jack so they were both solid black.
Rockbox is excellent because it runs off of a basic folder structure, but also has a library option.
You can do the iFlash mod and still use iTunes / the original OS if you like, but Rockbox has proven to be superior in all ways.
If you're on a Mac, be sure to format the main card with a Windows box so its Fat32.
(1) https://www.aliexpress.com/item/32816882257.html?spm=a2g0o.p...