Can really recommend PetNeeds (www.pet-needs.co.uk) - sorry not sure how to do a link. They are not cheap but use top quality english bridle leather. I have one that is about 15 years old and still in superb condition.
Niton will sell outside of the police. I order quite a bit of stuff off of them (im a security dog handler) another site i use is http://www.john-humphris.com/