I use eneloops with their included chargers with no issues.
If that's not good enough then you should be looking at external battery packs, not rechargeable AAs.
The problem with external battery packs is two fold. Most of the OEM's use 8 rechargeable AA batteries and are moderately expensive. The ones that don't rely on rechargeable AA batteries are just plain expensive.
In addition you still need batteries in the flash to use a battery pack. The battery pack supplies the power to recharge the capacitor, the internal batteries provide power for the electronic operations of the unit itself.