First, you need a Walbro K20-WAT kit and I recommend the genuine Walbro kits over the Chinese generic kits. Stens or Oregon brand kits are also acceptable but they're about the same price as Walbro so why accept substitutes.

These can be bought at any good saw or outdoors shop that sells gas powered tools. Many online hobby outlets also sell these and Valley View RC comes to mind. Ebay is my source of choice and there are many outlets there and many offer free shipping. Again, look for the genuine Walbro kits.