The best ones that I have seen are from B&B Fabrication, 888-228-7435. I used them with my B&B tri-y headers and they are very well made, waterjet cut stainless, and about .300 in thick. If you catch them in the right mood, they might sell them to you. There are plenty of carbon steel flanges around, including JC Whitney. For your pipe consider using Burns stainless mandrel bent pipe. It's the best you will find anywhere.