What type of bolt is only threaded at the tip
It is called a shoulder bolt. You will probably need to try a specialized hardware supplier. In the US, Fastenal or McMaster-Carr come to mind as far as national outfits.
With this type of hardware, it is sometimes so specialized to the item it came off of, it could be easier to just locate the manufacturer and model of the scooter. You could then first search online for parts diagrams and part numbers for that, or call their support number.