Exhaust Gasket, Greddy SP2 Options

[UTSlickVic]

Hi all!

Noticed my muffler sounding a but louder and a slight swooshing sound when hitting the gas pedal. Looked underneathe and saw that one of my exhaust gaskets is almost gone. Its the oval gasket that is at the pipe flange joint closest to the muffler. Its a greddy sp2 and wondering what's the best place to get a replacement?

Since it's a aftermarket exhaust, would a oem size not fit? Is there a size difference between the GT vs. ST size (I ask b/c I can't remember if the greddy was more sized to the GT). Greddy website has gaskets for purchase bit I'm not sure what size my sp2 is.
Looked on rockauto but all it shows me are round gaskets. Amybe there is another gasket at that joint ??? And I'm a bit thrown off by their termonolgy.
I may hit up Autozone or Orliey's just to see what they have.

Any useful info would be appreciated!

Here is a schematic of the exhaust. The joint I'm talking about is at #31320