You can search for such things but I find the usual USB charger thing that takes batteries is a fine solution. You may encounter folk that sob or complain if they don't get exactly what they want but what stops folks from ripping out the rechargeable and making their own AA battery holder? 2 wires, a little glue and the battery holder and we are done. Where have all the folk that fixed and made their own stuff gone?
Here's the thing I own -> http://www.instructables.com/id/MintyBoost!---Small-battery-powered-USB-charger/
You can find such a thing if you don't care to make your own.
Are there any current models of MP3 still using double AA batteries. For obscure reasons this is what I prefer and my old Cowon IAudio G3 just died or at least the on/off button is innow inoperative. Thanx for any help!