phone anyone who is a shimano service centre, it should be a stock item. if you look in yellow pages most shops that advertise in there should have it listed on their advert but any shop should easily beable to get it even if it isn't in stock cos every shop has a madison account.
most cones have remained standard for years coz shimano basically haven't changed their hub design since god was a very small boy!
the best thing to do is give the guy in the shop the part code of the hub, it should be written on the centre of the shell, he will beable to look it up in the big book of shimano and get you the exact same one(but new!) also, spares are post free to shimano service centres so he could order just 1 cone for you with no small order charge or postage charge. £2 or £3 depending on which one it is.
When I have needed a cone for old hub I have took cone off from the hub and I have gone to the lbs with cone. Some shops have several boxes full of cones. After some search from the boxes I/shop keeper have found spare cone.
translation,,,,all good bike shops should stock shimano cones,our`s did when i was there,and i just took one of an axle if i didn`t have one to hand and bought one in to replace it