I have Yorkies, had them for years along with my bigger Skyes.
There is no such thing as a "toy" Yorkie. All Yorkshire Terriers are part of the toy group as specified by the Kennel Club. The breed standard states 7lb as the top weight,& they need to be this or slightly under to win at shows, but many Yorkies turn out to be over this & make very good, more robust family pets. It's inevitable that you will get some of these larger ones, even if you mate two small ones, as Yorkies originated from much bigger terrier breeds, the Waterside, Clydesdale & Paisley terriers ( all extinct now) & the early Yorkies were all much bigger, around 14lbs, than now. Yorkies are a relatively new breed, at just over a hundred years old, so the size isn't fixed like in many much more ancient small breeds like the Pekingese or Maltese & so you get quite a big variation in size still.
To get a healthy puppy, much more important than just size, your very best route is to contact the secretaries of some of the Yorkie breed clubs, all contact numbers can be found on the Kennel Club website, & ask them if any members have litters due. You will find that once on the grapevine of reputable breeders you will be passed along to someone who can help you. You may have to wait, I know that with Skyes ( an At Risk Native Breed) we have waiting lists, & expect to be as interrogated by a good breeder just as much as you will want to interrogate them.