Can someone tell me where I can pick up the donut flange and gasket the bolts up on the cat attaching to the header?

1. I want to make a custom 2.5" cat, but I am not sure where I can find this part.
2. Will any 2.5" donut flange bolt up to the header flange?
3. Any know the outlet size of the header flange and inlet of the cat donut flange?
4. What is the header flange called?

Thanks in advance.