Just ring your local KTM dealer, they all keep them in stock, £8 - job done
If you have trouble finding 1 just ring Redline KTM, they have them in stock and can post you 1 for £12!
I no this cuz mine turned up yesterday
Now as for fitting, the easiets way ive found is to drop the system off from the front and rear cylinder head mounts, central mount and the mount where it connects to the silencers,this allows the system to move around, remove the front sprocket cover and a long bar with 10mm socket will release the clamp around the gaskit joint, remove old gasket, use a bit of WD40 to get the new gasket in and also to ease the fitting of the rear cylinder pipe back into the new gasket joint, realign everything back up, all mounts etc then do the final tightening of the complete system - should take around an hour