Remember to replace the gasket as well.
It might be worth replacing to short hose from the thermostat (only costs about a fiver) as mine split the other day and you have to remove the themostat housing to replace it.
Remove 3 bolts on thermostat housing, loosen hose clip then remove housing. replace thermostat, replace housing etc etc etc.
As Wrinx stated main Alfa dealers will probably not have parts in stock, try local Alfa specialist (if you have one).