I got my bars from a small branch of Boots. They were next to the slim-fast in the 'dietary foods'. Quite expensive but wow, when you've not had chocolate in a whole week the chocolate brownie one is amazing!
Dont forget the Boots offer at the moment. Buy one get one half price. Still bloody expensive, but if you are using them as a treat rather than a daily thing its not too bad.
They are great as a treat or if you are away from home and stuck for a legal snack, BUT beware because I reckon they can cause sugar cravings. I relied on them heavily when I had a weekend away recently and they were a godsend but I really suffered with cravings for sweet stuff in the following week, and I don't normally crave the sweet stuff AT ALL. It took a couple of weeks to shake them off.