First stop would be the breed club, the national one and the appropriate regional one if there is one.
You could try going to the appropriate breed section on Champdogs and looking under both 'Breeders' and 'Owners' to see if you can find the breeder.
Have a look for a reasonably active forum or FB group dedicated to the breed and ask there. Most breed forums/FB groups tend to have at least one pedigree geek who may be happy to help.
Were her siblings bred from by her breeder or did they go on to other breeders? If it was other breeders then you should be able to track them down by their kennel name which is usually added onto a dog's original kennel name. So it would be OriginalKennel Blah-de-blah at NewKennel or similar.
Do you have a MyKC account? You can look up individual dogs on there and get a bit of information about them. I don't mind having a look on mine if you want to pm me her full kennel name.