How do you know what to buy?

This is the front left view of the bicycle.  There is a plug located above the handlebars.  As you can see it says, 6V 2amp minimum.  What that means in electronics terms is that this bicycle needs no more and no less that 6 Volts of electricity.  The current of the supply need to put out a minimum of 2 amps or 2000ma of current.  That solves halve the problem of the power supply.  The other problem with the power supply is you don't know the polarity by the markings on the bike.  Polarity is very important,  this is the direction that you plug something in.  For instance, if you hook the jumper cables right on a car the car starts.  If you hook the jumper cables up backwards on the car, sparks fly and stuff smokes and possibly explodes.  Same with you bike if it's hooked up wrong.  I'll tell you what my polarity was on mine.  The center pin on mine was positive the outside was negative.  I have no know idea if they are all the same.  I'll show you how to check in a little bit.