The key for a Yamaha scooter is required not only for the ignition but also to access the lockable fuel cap, under-seat storage and rear wheel lock. The key is also required to access the lockable glove box in which most riders keep their state registration and insurance information.
Look at your spare key, if you have it. According to a Yamaha scooter owner's manual, Yamaha scooter keys have a "Key Identification Number" stamped on them. The "Key Identification Number" is a 5-digit alphanumeric combination such as A1234. You will need this number when contacting your Yamaha dealer to purchase a new key.
Consult your Yamaha scooter owner's manual for the "Key Identification Number" if you do not have a spare key. The Consumer Information chapter has a page where the "Key Identification Number" and the "Vehicle Identification Number" may have been written.
Call or drive to the Yamaha dealer where you purchased your scooter. Dealers are required to keep files with information for each vehicle sold and can look up the information needed to order a new key after you provide some identification. A key ordered through your Yamaha dealer is pre-cut to fit your ignition when delivered.
Replacement Yamaha scooter keys are sold in sets of two. After purchasing replacement keys, write the Key Identification Number in your owner's manual.