I found a 2-Bolt Exhaust Flange Gasket for a 3” application that might work for you. You might need to modify it to make it fit.
The link is below for 3" SCE:
This link is for a 2.5" from Walker:
This one is from SCE on a 2.5":
The OEM 2.5" rear muffler gasket is below:
See if these work. The Walker gasket seems more promising. It has a collapsing ring and the micro fibers to help seal the exhaust leak. Besides its a direct OEM replacement.