Have a look here:
http://stores.ebay.com/SAMAT-SAS-di-...QQftidZ2QQtZkm
I have dealt with this gentleman (bought a recharge for my existing TIM SIM), and all went well. He offers to sell you SIM cards at a reasonable price.
Read the information carefully, as you will have to supply him with the required information to obtain a 'codice fiscale', plus a scan of your passport to register the SIM.