Hi, as Nova says, there are two clubs.
The PTA (Plummer Terrier Assn) and the Plummer Terrier. The latter was set up after Brian Plummer was expelled from the original breed club. As founder/inventor of the breed, he took the decision to perform an outcross as an attempt to wipe out some of the faults that had started to appear in the original lines. For this he was expelled and set up a new club- the Plummer Terrier. So, you'll have to read up on each and decide which way you want to go, but either club will advise on breeders. Both have a website.