If you have no luck with serco, give George Mackensie a call 32064230.
There is not much he can't do. He has some British thingamebob bike that was missing a barrel, so he just MADE one
He will also make a sleeve if it cant be bought over the counter.
If serco guys cant put their hand on a sleeve/ part in stock then it gets a bit too hard for them really quick.
Something else he did for me which i thought was cool, i gave him a barrel and crank out of a KTM 125 '84 that had no listings for parts.
He put an rm125 '00 model piston in and rod kit for an Aprilia 125! Respect!