If you're in South London, Gonnellas at Catford will do the job for you.
there are two options this way:
1. Get them to supply and fit a new one. It will cost £200.00+ because of the labour.
2. You can also go to a breaker and buy a handle from an older model 147.. you only keep the lever - you don't change the lock part as the GT has no key barrel. It will probably cost you £40.00 as these guys aren't silly and know how much Alfa charge for a new one; but don't tell them it's for a GT!! Great thing is it doesn't matter which side of the car it comes from, if they're only changing the moving part and not the lock.
The old 147 handles are really well made, high quality solid aluminium alloy. The ones on the newer GT are not - they're plastic, which suprised me a bit... Also the newer hinges are a different design, much narrower at the hinge point and look like they are designed to break....?
There is a ( very ) slight difference in the colour, but you'd never spot it if you didn't know. Just lubricate the hinge well before fitting.
3. Is the diy option that I would not fancy - there's loads of ( breakable ) stuff to negotiate if you do the job properly, including removing the window glass. Still, if you want to save £100+ in labour.....