This style of ball flanges do not use a gasket. The problem is, they are really a one and done style of joint. After an initial tightening mating the ends shapes them to one another. Try to unbolt and attach new, you're likely going to leak. Further, in stainless pipes, they don't seal well from go because the hardness of the stainless.
Exhaust leaks are considered acceptable beyond the catalytic converters and past any point of potential fume migration into the passenger compartment. So, you may not get anywhere trying warranty.
You'll get lots of bad advise of using a copper RTV, but just standard heat expansion will break that seal and it'll leak again. Aluminzed antisieze or exhaust paste lasts a little longer, and just skip the old-school use of high body grease because, well, it just stinks when hot!
If you want to really seal a leaky ball flange you can't tolerate, you need an "exhaust olive" of proper size. This is not a "doughnut seal" and has a different design that you can really capture within a ball flange to seal it. Though, a fine profile doughnut of the right size and geometry could work with longer bolts too. You're other option, which is definitive and more permanent is to cut out the ball flange and weld in a coupler. Understand ball flanges are used primarily to better accommodate changes in alignment of the exhaust pipe to move around things and put the pipe where you want it - like centering in a bumper opening for the tips. So, not a big deal to lose them. However, ball flanges with spring-loaded connectors are purposely done to accommodate movements expected. I'm sure many have seen this connection from manifold to down pipes. So chopping out one of those needs to have an alternative means to accommodate expansion such as a flex coupler.
