I'm a big fan of Wemoto, simply because you can see if stuff's in stock online, and their delivery is either normally next working day, or 2 days at most, which is important when you're trying to get things done.
http://www.wemoto.com/bikes/Yamaha/I used to use M&P a lot but they let me down quite a few times by taking your order when things aren't in stock, it's very annoying.
And i think their website is bloody terrible to use.
They also trade under the name 'Busters' as well.
http://www.mandp.co.uk/http://www.busters-accessories.co.uk/I've used Fowlers in Bristol a few times before, they hold a very large amount of stock, and are quite helpful over the phone too.
They'll always ask you for a chassis number before anything so have it to hand, but if other people haven't got what you need the chances are they'll have it, although they're a little pricier than most but you can't have it all ways. If it's in stock then delivery is quick as well.
http://www.fowlers.co.uk/parts/from_fowlers_web.shtmlThen as a last resort you have AJ Sutton.
A great site to look at for parts because they have diagrams of your bike broken down literally to every nut, bolt & washer which is very handy.
Only downside is that they're bloody expensive, some of the prices on there will make your wallet wince.
http://www.ajsutton.co.uk/