A good place is to start is often the main dealer, Yamaha, Honda, Suzuki. There not cheap as a rule, however you can be surprised so it is worth a try. Here are I have used and there not all main dealers.
Woodford Motorcycles Yamaha Maindealer
51 – 53 George Lane
Tel 020 8989 3157
Woodford Motorcycle seem to carry a lot of parts, Givi bits as well as clothing but its a little limited as clothing goes.