You won't find a pedigree 'teacup' Yorkie because breeding for miniature status like that is not a practice approved of by the Kennel Club. I would have thought that you would struggle to find a responsible (non pedigree) breeder who advertised their dogs as teacup varieties because there is a real stigma attached to the term and the 'fashionable' desire for teacup dogs. That's not meant as a judgement on your preference, just a statement of how the dog world views teacup sized dogs.
Yorkies are pretty small dogs, anyway, with some variation in size so you might do better to just focus on Yorkie breeders and check out the mum/dad size of dog when making a selection.
I found using a combination of the KC assured breeders list, champdogs.co.uk and plenty of independent internet research into the breeders listed on them was the best way to look for my own dog.
I cross referenced the KC and champdogs breeder lists to come up with those that appeared on both as active, then contacted everyone on the shorter list to enquire whether or not they were planning litters later in the year.
For those that replied to say they were I started looking them up on the internet. Basically, I was looking for good, solid, reliable history of their breeding lines. The breeder I chose had multiple news articles about her and her mum's dogs so I could trace her dogs back several generations. I could use the KC and champdogs sites to look up all parent, grand parent etc health checks. I could find online pictures for all those dogs that matched her own website to verify they all existed. I could see on her Facebook page a convincing history of responsible dog people she knew, plus an occasional update from people who'd had previous puppies from her.
Ultimately, though, I spent 90 mins or so face to face with her chatting about her dogs, her experience and the way she chose how and who to breed and when and why she had a litter. She let me see the contract up front before I made any kind of commitment. She was clear about what she expected from me as an owner and what I could expect from her - e.g. she expected me to contact her if I could not longer keep the dog for whatever reason, at any point during his life and she would take him back and keep him or find him a new home; in return, I could call her at any point during his lifetime for help or advice about the dog. During that visit she was open and allowed me to spend plenty of time with mum, dad, plus some other dogs she had from the same lineage so I could get a good sense of the temperaments of multiple dogs that would be related to my puppy. he did not push me to commit to a dog and just allowed the process to progress slowly and steadily.
Out of 6 dogs I've had, he is the only puppy I have bought from a breeder and, at seven months old, I am very glad I spent all the time doing the research because he is the picture of mental and physical health. Well worth it.