The standard plug, C7HSA or CR7HSA is very common and used on thousands of different scooters. Most scooter and bike shops will have them. They're so popular that even AMAZON have them:
http://www.amazon.com/NGK-C7HSA-Stan.../dp/B000CSGX5G
The iridium ones will only be stocked by performance parts dealers, as they're expensive to keep in stock, around $12-$15 .