If you already breed labradors and rottweillers then I would assume you would have the right contacts to find a well bred Border Terrier.
It is not that difficult to find reputable breeders, especially by word of mouth.
Your first port of call if that fails, should surely be the Border Terrier Clubs, which as I breeder I would have thought you would already know.
If you would like to tell me what area you are from I will willingly give you the telephone number of the secretary of your nearest BT Club.
But be aware that as a breeder of two breeds already you are unlikely to obtain a pup very easily from responsible breeders.