If I forget the fancy and expensive Japan stuff, only Wiechers makes rear strut braces for 147.
They do actually have 2 designs.
First one is simple bar with two flanges and you need to drill the suspension turrets and bolt it on. Its available in steel and aluminum and is quite cheap:
Alfa Romeo Shop - Tuning, Styling, Fanartikel und Modellautos - Wiechers Domstreben hinten Wiechers Domstreben hinten 48422-48423
Second one is from the Racing line of Wiechers strut braces. Available in Alu or carbon, but the prices are 2-6x more than the first option and the probelm is there are no real pictures. I contacted Wiechers about a year ago and asked them if they had any, but they replied that unfortunately not
and confirmed you need to drill the body as well to fit it. So buying it is a little lottery as you have no idea how will it look and install once it arrives.
But I have seen them offered in many shops in Germany, here is the alu one:
Fahrwerkstrebe Alu Racingline hinten Alfa 147 / 156 / GTA / GT | eBay
and here the carbon one:
W2 Fahrwerkstrebe Alu Racingline Carbon hinten Alfa 147 / 156 / GTA / GT | eBay
147GTA,Grigio Metallico,Imola leather,3.75l conversion,ported heads,F575 TB,Carbon plenum,Stainless manifolds,EQ pipes,Supersprint silenced cat back,18" GT multispokes,KW V3,Galfer 6pot 355mm,Brembo sport rear
147 1.6TS, Nerro,Beige Interior,Galfer Extreme Brake kit,2,0TS intake manifold,Custom ex. manifold,200CPI cat,Supersprint silenced center + OO backbox,GTA spoiler,GTA side skirts,GTA headlights,Blaupunkt LA MP74 + Focal + MacAudio + Dynamat + HERZ subwoofer