http://www.gearsandsprockets.co.uk/ - this is the spin-off from Technobots. No really specialist stuff but perfect for our needs. Never ordered from them as the new site but Technobots were always fine.

I often use http://www.beltingonline.com/, typically a little pricier but they have a huge range, including nice timing belts. Good service, can recommend!

edit: and there's always Ebay.