D&D Fabrication sell a flame cut set of flanges with 1.5" starter tubes or a one piece flange with the same starter tubes. These are made for the Buick 300 but one exhaust port can be removed. They also sell headers but I'm not sure if for the V6.
D&D Fabrication
8005 Tiffany
Almont, MI
48060
1-810-798-2491
The factory made headers for the 3.8 Buick Grand Nationals. The aftermarket has lots of availability for flanges and complete headers, probably some for turbo setups too.