Note that a 9V battery PACK is not the same as a 9-volt battery. You'd use 6 or 8 rechargeable AA or AAA cells in a holder. And for running motors, rechargable is the way to go, unless you really like buying non-rechargables by the case 
8 cells is probably a better choice than 6, since a NiMH cell puts out around 1.2 volts, rather than the 1.5 of a non-rechargable alkaline cell.