Welcome to the forum Johanne.
You do not say if your dog is Kennel Club registered or not? If it is you can contact the
kennel club and for a fee, they will send you a three generation pedigree. If you don't have KC papers, then it might be a bit harder to trace and could take a lot of digging. Again the KC might be able to help you through their breed record supplements. The only other thing I can think of is if you contact the breed clubs.