In the past few months, we have covered a bunch of cool robot gymnasts by hinamitetu. His latest one is a gymnast robot capable of performing backflips. It stands 13.5 tall, weighing 178g. The robot is driven by 4 x 200 RPM gear motors running on a 1000 mAh battery.
According to the developer, you can easily buy the battery and motors for this robot on Amazon. More robots by the maker can be viewed here.