Had the same issue with my Alfa 147 - in theory you're supposed to replace the entire thing.
As Jim quite correctly identifies you can get the replacement rubber blade, tends to come in rolls and you just trim to length and apply to existing arm. I've always found that a bit of a fiddle though.
If you have a look on good old eBay you can get the replacement around for less than £10, it's an easy change. Just flip up the little cap on the end of the wiper arm, undo the 10mm nut on it, pick a swear word of your choice - wiggle the arm and off it comes. Apply a tiny drop of copper grease on the splines of the motor spindle as you apply the new one (align to roughly the same angle as the previous arm) and re-fit 10mm nut.
It's genuinely a 10 min job