I use Mania or Heaven as mentioned above. Mania is cheaper on postage if you are only ordering a small item like a spring or something, and Heaven has a nicer website if you want to purchase through an online store instead of email then paypal. Both are brilliant suppliers and contribute a lot to pinball shows every year.
I tend to opt for Mania most of the time - I like his underdog of a website
Support the U.K. suppliers first, both Pinball Heaven and Pinball Mania have most spares that you will be needing. Both have a great service and are great guys as they both support pinball events in the U.K.