Dunno where you are at but MOST dealrs stock them, a bimetallic and fiber asbestos type gasket. cheap usually for about $4.00 each but id have to look it up to see. Not a bad thing to have sitting on the shelf anyway as if you need to work on the bike you never reuse the old ones. Anyone with these bikes should consider stocking basic service items and buy ahead of time. Filters, gaskets, and basic consumables,
check for loose or stripped or broken exhaust studs, common on XBs and Sportsters, if damaged theres MANY threads on how to repair, but once leaking you should fix ASAP, a air leak into the exhaust at the port is like a flame thrower,, adding ambient oxygen vastly increases temps and can burn valves or other parts plus really damages the pipes.
On cars they use Air injection just past the exhaust ports on the pipes as a emissions control device to burn off unburnt fuels,
