In the UK, pretty much the best place to go is Pinball Heaven, great knowledge and pretty helpful. The best (although not necessarily the quickest) place to send a Pin that needs a repair.
http://www.pinballheaven.co.uk/shop/index.php?language=en
The other place for great prices on parts is www.pinballmania.co.uk - the buying procedure is somewhat antiquated to say the least, but the bit's are well priced and once paid for delivery prompt.