Go to the wiki (link up at the top of the page there.) Search for "tires". I added my tire link collection to that page, so you should have plenty of sources to check out. If the shipping charges seem exorbitant, you should call the company and see if that's the real value; a lot of times they estimate based on motorcycle tires, not smaller, lighter moped tires.
You can also try to find some locally and save on shipping -- most local motorcycle or powersports dealers should be able to get you some.