Only Agusta? Hawker Pacific should be able to get them for you, or Aviall. Make sure there is nothing in your purchase contract that says you have to purchase all your parts from Agusta. There shouldn't be though. As long as the tyre has the same P/N as specified in the IPD (DR9837T) you are good to go. That P/N is a Dunlp P/N, and is readily available at aircraft tyre retailers. It should work out cheaper for you too. As I said though, make sure you're not voiding any parts supply contracts you may have with Agusta.
For K1W1 land, try Flightline, or Airwork as well as Hawk Pac and Aviall. And don't worry about spare nosewheel tyres, you won't need them for a looooong time. Best thing to do for your mains, is to have a spare set built up in stores. Instead of frigging around flipping the tyres to even out the wear, change the whole wheel, then you can flip the tyres in your workshop at your leisure, and put those wheels back into stock. If you have extended taxiing to do, pickup and hover taxi to save the tyres. Until Agusta fix the geometry stuff up that they made with the axles, that is the best that you can do. And carry a couple of Rotor Brake Actuators too. They don't like the heat in the compartment where they live, and are continually crapping out. An 'improved' actuator is supposed to be released soon.