Nowadays it's much easier to purchase tyres on the internet. This pre-supposes of course that you have the skills and/or tools to fit them yourself! Shop around - but make sure you check delivery costs as well; with these added to the price of the tyre it may be cheaper to take the bike to your local tyre dealer.
Warning:
Once your new tyres have been fitted, remember that they will still be covered with a layer of mould release agent from the factory. You may not see it, but it's there. In order to wear away this layer, tyres need to be 'run in' before they can be expected to cope with hard riding or braking. So ride smoothly and slowly for the first 100 miles - and your tyres will then be ready for whatever the road may throw at them!
Here's a few places you can check:
UK
MyTyres.co.uk
Black Circles
Wheelhousetyres
If you're in the Berkshire area I can recommend the service from Ride-In.
