If you are talking about something like the Goodwin super q silencer it uses a flat sandwich type steel and paper gasket. I tried 2 options.
Order a replacement through the racing beat europe website after contacting them on their enquiry form.
Order a sheet of BGA Rpk 1 gasket material on ebay and make your own. Not as easy as it sounds as the metal and gasket layers tend to delaminate while cutting.
The bolt centre holes on the Goodwin flange are 41/2 " and the the centre hole about 61mm. This may be a common size in the US but I couldn’t find anything off the shelf from Walker exhausts catalogue.
If you use the original ring type gasket with a flat aftermarket flange be wary of distorting the flat flange when tightening the joint.
It’s for a Cobolt exhaust and the aftermarket sales for that stuff in the UK seems very poor at the moment. Probably just best to get it and measure up and go from there.
I used the crush gasket linked to (Mx5parts) to fit my Cobalt back box. The flat washer type gasket that came with it didn’t seal very well.
Just offer up the back box first (fit to hangers) to make sure it sits square before putting the gasket in. The mid section pipe with flange has a protrusion, the gasket fits nicely over this, just clean it up so it sits flush, mine was a little made up with corrosion and muck etc
See if you can find some red rubber grease or silicon grease for the rubber hangers as it makes replacing and subsequent removal of back box much less of a hassle.