I've been looking for a while and haven't been too lucky, cheapest I found was a special order from wemoto, about £50 new, and £16 for the gasket (don't forget that!!!).
Depending upon if you have a hole in it, clean the outside with a cleaner like brake cleaner (something that doesn't leave greasy residue) and plug it with araldite.
Sounds like a bodge ( but it keeps you on the road!!!) but did 1900 miles and counting and no leaking from the case.
