Hi Laura,
Have a look in the classifieds for car breaking. Also check local scrap yards and E-bay. They are also available from Fiat, but you can pick one up for a few quid from the above.
Hope that helps,
Alan
EDIT: Oh and as by the sounds of it this has happened to you before, check the condition of your heater fan motor. Check it spins freely and is not getting flooded with water from the scuttle panel. Use a spray lubricant on its bearings. These are the common causes of resistor pack failure.