To any exhaust shop that you trust, so not to mess with the resonator barrel.
EDIT: As I was looking for the same, I went to several shops and I decided for this one:
http://www.tsoukalasexhaust.zoomshare.com/
He is at Ilioupoli, has done it before and he knows.
I will go propably next week. If you want you can wait to see photos of the result.