But for now if my player has some USB cable I use my USB wall charger on all of them without any ill effect for many years. That may not be a good answer for some folk but that's my choice.
I have been wondering if it would be ok to charge an mp3 with plugs of mine like for my phone that have a USB slot for charging something. I have a bunch of plugs like that with just a USB you can plug into to charge something like for my Bluetooth and one for my smartphone etc. I also have a bunch made for car power outlets but I'm afraid to because I'm afraid if its not the right plug than it might do something to the device or to its battery or something. I don't know.