Try Maplins, they stock most of the Antex range at all of their branches including the 15, 18 and 25watt versions. they also do a good range of replacement tips.
www.maplin.co.uk
Try to find a store close to you, (they have over 60), if not then try their mail order system. They also have a good range of soldering accessories available. I might be praising them a lot but i did work for them for over 11 years.
Give me a shout if you get stuck and let us know how you get on.