There is one that I think is just called the bit bank. I have used it quite alot and it offers a good service and a lot of bits. You can have most of the bits on trial as well.
Hope that helps.
Personally I do NOT recommend http://www.thehorsebitshop.co.uk/, shocking customer service. I'd rather have paid upfront for the bit and got a refund than go through the 'trial' with them.
horse bit shop = horrdendous (nightmare to contact, never answer phone/emails. Don't tell you the bit you want it out of stock and you have to wait WEEKS for it!)
bit bank = brilliant!! Fast delivery and so helpful when you ring. Let me keep hold of the bit I had on trial until a new version of it came into stock for them to swop over as I wanted to keep it. Will definitly use again!