The internet is a great place to source discount motorcycle tires, but a good old fashioned shop has its advantages. The tire prices might be higher but they will probably fit them for you, using proper tire changer tools, and professionally fitting motorcycle tire balancers. Some garages won’t fit tires that you’ve bought from elsewhere, but that’s down to the garage.
Online, the best resources are Amazon, Revzilla, and Chapparal Moto (Chapmoto).