I've used bits & kits (
http://www.bitsandkits.co.uk/).
Like other bits sites, however, you sometimes have to be lucky to get the bits you want as they sell out of the 'good' stuff quickly.
They do send out emails alerting you to when they re-stock which is good. If I had 3G on my phone I would probably be happier as I could get in before everything disappears.