Something I would be cautious about is the inductive nature of any electric engine. Switching off a live circuit with inductive components will result in voltage spikes, and therefore electric arcs that can easily kill any switch and possibly even damage other components in the circuit. I recommend you watch this video for further information on that topic.
Before trying to install a switch, I recommend trying to reverse engineer the power electronics in your scooter to make sure there are adequate protections against this, and if not, adding a flyback diode to the engine should probably be enough to protect the circuit. I assume you are an EE student? It'll probably be a good exercise to try to figure out how your scooter works, just make sure to be safe and disconnect the battery before doing anything.
Now, as for selecting an appropriate switch, you can search suppliers like digikey and do a parametric search for the appropriate voltage and current ratings. However, depending where you live, shipping costs will probably be very prohibitive and it'll probably be much cheaper if you just search for local electronics suppliers and personally ask them if they have switches that meet your criteria. You can always use ebay, but be on the lookout for dodgy parts.