should i buy a yorkshire terrier or adopt one?

Adopt
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If your planning on buying from a respectable breeder who shows and health cheeks their animals go ahead.

Other wise their is normally nothing majorly wrong with animals in shelters allot get handed in because the owners wanted a cute little dog and didn't no how much work it takes to raise a dog.

Other don't have the money anymore to look after one.

Shelter animals are normally cheaper and should come spayed/ neutered and vet cheeked. So If you want the cheaper option but still realise that dogs cost money a shelters would be your better option.
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Adopt - you can contact a yorkie rescue group. Just google it. My best dogs are ones that I rescued. They don't understand why they were treated badly even though they loved their first family. They seem to want to compensate and love their new family even harder.
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Human children are adopted-animals are bought & sold!

Your choice as to where you buy an animal….IF it's an educated one.

Spending(possibly) more up front from a reputable breeder who'll stand behind their stock for the rest of it's life OR a "pig-in-a-poke" unknown & un-guessable dawg from a pound???

THINK carefully;don't just react *OR* be bullied into something you don't really want.
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No no…not all shelter dogs come from abusive or neglectful situations (the recent forclosure crisis has left tons of perfectly healthy dogs in shelters) and most are quite healthy. When you adopt from a shelter or rescue, the dog will be fully vaccinated, spayed/neutered, which saves you time and money in the long run. In general, if you are looking for a pet and a companion, a shelter or rescue is the best thing to do.

If you want to show the dog in conformation, then you're better off purchasing the dog from a reputable breeder. Otherwise, the shelter is the way to go.
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Adopt one….
Just because they are in a shelter it does not mean that the dog has something wrong with it. Many times they are dogs that have been rescued from backyard breeders or puppy mills. All shelter dogs should come vet checked and up to date on all shots as well as most shelter provide a health guarantee (usually something like 30 days)
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PLease adopt! Most dogs that are brought in to shelters have nothing wrong! Their former owners just didn't want them. Please there are thousands of dogs in shelters that need good homes with people like you. And please don't buy puppies from pet stores they are most likely from puppy mills and buy buying the puppies you are keeping that puppy mill thriving. So please adopt!
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Shelters don't adopt out dogs with broken legs, I've adopted shelter dogs all my life and we have never gotten a 'broken' dog. Lots of dogs are rescued because nobody wants them due to there being so many homeless dogs out there. Your better of adopting a shelter dog, they will have been temperament tested, have all there shots and if old enough will be spayed or neutered.
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You're not like lo find a purebred Yorkie at most shelters. Adopting would be a great idea but you'll probably need to do so through a rescue group. Just please do not support any backyard breeders even if one wants to sell you a pup for what seems to be a very good price.

http://www.ytca.org/frame_index2.html This is a link to the rescue resouce page of the YTCA (Yorkshire Terrier Club of America) website.
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Adopt. Do you ever watch Pet Star? All the winners were adopted from a shelter. Plus, some breeders lie about shots. My friend bought a dog, and about two weeks later, the dog died from parvo; the breeder lied about shots. The shelter dogs are much sweeter, I wouldv'e gotten a shelter dog if my friend hadn't given me one for free.
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Many animals in shelters are there because of behavioral reasons. However, you will not find a Yorkie at the pound. You will have to contact a breed rescue, jump through many hoops, and pay a large purchase price (prob like $500). They call it an adoption fee, but really they are selling you the dog. It doesn't cost $500 to spay a Yorkie or to feed it. Also, hope that they tell you the truth about the animal and its behavior problems. I would suspect housetraining problems with any young Yorkie at rescue.

A friend once described a dog as a black box: you get out of it what you put in. This doesn't mean that they are objects, but rather that they are easily influenced by their owners. The question is do you want to deal with all the problems a rescue has? Or do you want to start fresh with a puppy and train it for your lifestyle? There are advantages to both. For me, I like to train my own dogs. I don't like to have to deal with other peoples mistakes. However, sometimes you can get a really nice dog at a rescue. Also, you can avoid all kinds of puppy problems. Yet, we have a saying in dog training- better no training than bad training. In other words, its more difficult to fix mistakes than it is to start fresh. JMO

Good luck!!!
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30 yrs dog experience and training