got mine at this place on ebay:-
http://stores.ebay.co.uk/thegasketshop
did it myself, never leaked since.
as you smell burning oil, I assume it's the drivers side one that is going ? Easy enough to do. remove airbox for easier access and you will need an offset ring spanner to get at a couple of the bolts further back.
if you have a classic, I think there's 2 different types of gasket sets depending on the heads (Tone will probably know how to tell the difference).
The only tricky bit is that once you have the new rubber seal sitting in it's grove, there isn't much room to get the cover back on and you will probably knock the rubber seal out of it's grove (Took me a few goes) so check that before you tighten things down and damage the rubber gasket