You may get some suggestions to buy LiPo batteries. They perform much better than the NiMH battery you currently have, but they are also a fire hazard and they require maintenance and care to be safe. If you just want some easy batteries that work, get more NiMH batteries. Replacements are available on Amazon for about $40. Just be sure to buy one with the same plug as your current battery so that it will fit your car and your charger.

You might also do a little research on how to revive a NiMH battery. I think if they aren't used for awhile they can get weak but if you cycle them a few times it may revive them back to full capacity. Your existing battery may actually be fine, it just might need to be cycled a few times (charged, drained, charged, drained).