Where I can I find the mating header flanges so I can bolt on the reducers?

Ok my header pipe is 3.5" diameter, with no flange on it, just a round pipe. I got the header reducers to go from 3.5" to 3", since that's the size exhaust I'm going to run. The reducers have the three-bolt hole flange, but nothing to bolt on to because the header doesn't have a flange on it. So how do I attach the reducers to the headers? And where can I buy the part?