This is my thinking where does one get Oettle mod done?
This muffler came from Germany, as Oettle is a german company; however, any competent welder can handle the modification. I happened to purchase mine from a fellow owner who paid the freight from overseas, but didn't install since he ended up selling the car for a 996.
If you search here and
www.renntech.org, you'll see a number of owners that have successfully made the modification.