You can get the batteries at Fry's Electronics if you have one, or the local electronics store. If you want the same speed go with a 6V, we got 7mph out of a 12V, or go all out and get an 18V for max speed. The motors are rated at 18V so I wouldn't do anymore than that if you don't want to buy new ones. Make sure you get a battery with a pretty good Ah rating or it will go dead fast. Anything over 10 Ah will be fine.
As far as the high speed lock out goes, his just has an auto brake when you let off the gas pedal. Is this what you mean? It's kinda cool cause every time he stops he does a sliding 180 hand-brake style skid. Since I put the rubber spray on the tires this has gotten a lot better. If your daughter is good at it, I would definitely recommend the 12V battery, not too much power and you can leave the tires plastic, but you still get more speed.