I've seen they sell 24 volt e-bike motors cheap like around $50, so you could easily buy one and a deep cycle battery, then do your own conversion from a pedal bike.
Personally I'd like to see them make electric rollerblades with retractable wheels, so like you're walking along and want to glide on down to the bus stop, out pop the wheels.
Lol! That's a cool idea...don't give it away! You could make a million.