where can i find a miniature yorkshire terrier for $200?

Yorkshire Terriers come in one size,there are no "Toy," "Mini," or "Teacup."
Check local shelters/rescues if they have any Yorkies up for adoption.
petfinder.com is also an excellent site to search.
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Nowhere. Yorkshire terriers come in ONE count em ONE size only and that size SHOULD be 7 pound and under but pet quality and byb bred yorkies can run bigger.

They don't have a MINI size and no one can guarantee you adult weight.

You want a 4lbs dog? Go adopt an adult from the shelter.
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The pound
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your local BYB there is one in every street

seriously check out your local shelters
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Your Answer:
There is no such thing as a Miniature yorkshire terrier. You might be able to find a yorkie for 200 on petfinder.com. Keep in mind that a dog is expensive and even if you find a dog for only 200.00 you will still need to pay for an initial vet visit, shots, spay/neuter, micro chip, fecal test, heart worm test, heart worm preventative, flea preventative, supplies, emergency money, etc. Usually the cheapest part of a dog is the initial cost, so be prepaired..
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There is only one size Yorkie - so you would only find a miniature or toy or teacup Yorkie from a backyard breeder or puppy mill for that price (some charge you far more).

A well bred Yorkie costs more because the breeder has taken the time to test the dogs, show them and guarantee health.

You might find a Yorkie for that price on Petfinder.com or a breed rescue group. But you won't find a "miniature Yorkie"
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You can't. For starters, there is ONE standard for the YT, no subcategories for "minis" or "teacups" or "micros."
Secondly, if your budget is limited, your best bet is to check with rescues and shelters as you will not find one froma reputable breeders for such a little amount. Breeding ethically is not cheap. Breeders have costs to cover–food, stock, vet bills, vaccinations, medical tests, grooming and whelping supplies, registration fees, etc.
Your wisest move would be to focus on finding a reputable breeder and not so much on price.
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Yorkie breeder 10+ years