Where can I buy this part ?

There are two sizes for the pinion nut. Depends what car you have?
the illustration you show above is from a very good German firm I often use, Bertelsbeck Alfa - why don't you just get it from them? (Pinion Nut = Mutter Antriebsflansche in German, that'll keep you busy) they seem to have both the small one and the large one, depending if your car is a 1600/1750 or 2 litre. 40 bucks for the expensive one, 15 bucks for the other! [EDIT: that includes 19% german purchase Tax, which you will not have to pay, being in the States, or at least you will get back]

IAP list the 2 litre one for about 55$, but availibility is "please call" so might not be in stock
Alfa Romeo and Fiat Differentials and Components

You will need the Pinion nut tool to take it off and torque it back, so if you need it, try either "totallyalfa" who are specialised in making aftermarket tools:
Pinion/Crankshaft Nut Tool - A.5.0114 (A.5.0114 ) :: TotallyAlfa

or this guy (if he still is around) on alfabb:
http://www.alfabb.com/bb/forums/alf...67588-71-93-alfa-romeo-pinion-nut-socket.html

or make your own!
The Art of Tool Making or, How to Make a Differential Pinion Nut Removal Tool

hope that helps and good luck!

Thank you so much for your reply, it helps me !

sorry, didn't realise you were in Europe........

I looked in Europe, Victor Parts in France sells the Pinion Nut Tool and of course the nut.
The nut price is lower, about $18, but the tool more expensive : $93.

I think I will try to make my own tool.

Victor Parts have many many parts for the 105/115, but prices are sometimes amazing !

Victor Parts is basically Bertelsbeck. Must be a sister company as they link to the same catalogue.

Socaro too, but prices are higher and they don't have a big stock of parts...........INFO ou INTOX le future en Alfa Romeo: scoop, nouveau modèle, projets et concept car