Yes, though your limitation may be the physical space that the battery pack takes up in the car.
Nickel Cadmium batteries are slowly being phased out of the retail market and being replaced by Nickel Metal Hydride (Ni-MH). All the cell sizes remain similar to the ordinary alkaline cells, e.g. AA; AAA; C etc.
All are still available from Maplin < www.maplin.co.uk > and check out batteries.
The individual lead-acid batteries that I remember went by the 'Cyclon' name and marketed in the USA. They were 2V each and to get 10V all you needed were five wired in series, but they were on the large size.
Using 12V on a 10V motor would not normally be a problem as you would usually be going at full power for only short periods, the limiting factor might be the speed controller; can that handle 12V?
The newer cells on the market are Lithium-Polymer (Li-Po) and three cells in series would work fine. They have far more storage capacity for the size than all the other cells. However, you would need a specialist charger unit for it, under NO circumstances use a Ni-Cd or Lead Acid charger. West London Models < www.westlondonmodels.com > should be able to answer all your queries. Good luck.