You can purchase spare parts for the Aprilia RS 660 online from various retailers. Websites like Moto Machines, Ultimate Parts, and Aprilia Special Parts offer a wide range of genuine, OEM, custom, and performance parts. These include accessories such as MIVV exhaust systems, Bonamici rearsets and top triple clamps, Rapid Bike tuners, and braking system components like Brembo brake pads. Additionally, maintenance kits with different oil options like Motul, Silkolene, and Castrol are also available. These online stores provide detailed catalogs and often offer free shipping on orders above a certain amount, making it convenient to find and purchase the parts you need for your Aprilia RS 660.