I don't know about $10 a tire, but there are plenty of online options. If the shipping quote seems exorbitant, call and ask. I think a lot of places estimate based on motorcycle tires rather than smaller (less expensive to ship) moped tires.
"Ron Ayers": http://www.ronayers.com/
"Dennis Kirk": http://www.denniskirk.com/jsp/common/Frontpage.jsp
"American Motorcycle Tire": http://www.americanmototire.com/catalog/index.php?cPath=362
"Tires Unlimited": http://www.tiresunlimited.com/default.htm