pinions and batteries

[tamiya monkey]

hi there can anyone help i have just got a hol buggy and a pajero the wheelie one and cant find any good hump back batteries only second hand ones. my newer cars all run using 3000 mah nimh packs can anyone help in showing me how to make up battery packs so i dont have to modifiy my two older chassis cars to make new packs fit. also i am strugling to find a pinion gear for both the holiday buggy and pajero so if anyone can help with this. is there a modern one for a newer buggy that is the same.