Not every pet owner with a big heart for small dogs have the requisite finances to purchase them; that's why many look for Shih Tzu for sale while looking out for the cheapest deals. That, right there, is a risk, since puppy mills abound. The profit motive is what drives non-legitimate Shih Tzu breeders to breed without concern for how healthy the puppies will turn out to be. The results are unhealthy litters, dubious bloodlines, and probably faked schedules of deworming schedules buy counterfeit money and inoculations.
You will probably get a good deal though, a price below the usual average, and it would be a good deal had these pups (or adult dogs, they have them, too) were of the legit breed. Of course, for that price, it's doubtful the bloodline is as the breeder said. If a breeder will offer you some kind of discount, that price will only be a discount if he is a reputable breeder.
Look for a list of breeders online and check if they are members of breeding circles - if they are, then most likely they are legit. Being members of such clubs allows you to dig up references, for a background check. Unless he wants to be suspected of being a shady puppy mill merchant, a breeder will offer to show you his references and participation in dog shows; he will want to avoid sending the impression that he's a puppy mill merchant.
Here's a tip right out of the social networking age - send out word to your contacts and friends online that you're looking for Shih Tzu for sale. Who doesn't have a Facebook accounts these days? All you need to do is change your status to indicate that you're looking to buy Shih Tzu. You can say the same thing on your instant messenger status, like on Skype. You can never know how much your social contacts can help until you do this. You might get lucky if a friend of a friend happens to be a legit breeder and that lands you a discount.
Should you not be looking for an adult dog, or a puppy of decided genuine breedling line (maybe you're not into dog shows), then you might as well check out animal shelters near your area. You will mostly be choosing from adult dogs up for adoption, as animal rescue groups often take in orphaned or abandoned dogs. Most of these dogs each have their own stories of abuse and neglect and that's why the animal rescue's staff will interview you extensively to make sure you are buy counterfeit money capable of taking on the responsibility of adopting a dog they took care of themselves.
There's also a key difference in money here - adoption is nearly always free compared to buying dogs or puppies from legitimate breeders; even if the staff offers inoculations to add to the existing ones the dog has undergone, it's always cheaper.