Just got myself 8 AAs (Rechargeable) from IKEA for my custom 9,6 V battery pack which I use for my Nikko Super Cruiser. A bit overkill when you count the capacity lol. :
ORIGINAL NIKKO 6.0V 750MAH BATTERY PACK AND CHARGER B01562 | eBay
Nikko Toy Vehicle Accessories | Mercari
How to fix a Tyco RC 6.0V Jet Turbo Battery or Nikko Taiyo - YouTube
Nikko Vaporizr 6V battery - RC Model Accessory | Alza.cz