I had my exhaust custom bent and installed by an exhaust shop some years ago. its long gone out of business and quite frankly, I wouldn't do business with the guy again anyways. That being the case, his work was really top notch. I noticed it louder than normal the other day and theres a big hole blown through the Flowmaster. The exhaust is 8 years old. I'd say its overdue.
Here's my issue. I was looking at getting a stainless system from Summit Racing this weekend but the flange that is on the end of the Catalytic converter is missing so theres nothing to bold the new system to. Apparently when he put the exhaust on he cut the flange off and welded the pipes together. So I'm not sure what to do now. Can I just get a flange and weld it on there? I'm not sure what type of exhaust gasket was used there, flat or donut? I'm at a loss here. I'm betting theres a relatively simple fix for this but I don't know what it is lol

 

The flanges rust out, so yes he probably cut out the old rusting one and weld pipes together.

Yes, you could get a new flange and weld it in place, these often used a "spring bolt" setup and a donut gasket to allow them to shift slightly without leaking.

Since burning gasoline(H) with oxygen(O) produces a by product of H2O, exhaust systems rust from the inside out, always have.
Stainless steel won't rust, at least not very fast, lol, so it is a good choice for exhaust systems, and why most car makers have used it in recent years and advertise it as "lifetime" exhaust, Mufflers and Cats still rust from the inside out