Check out the rescue group and see if there are any complaints. You can call your local SPCA to do this. If they confirm that the rescue group is a upstanding group, I would go ahead with your adoption.
I want to alert you first: Puppy mill dogs MAY have medical problems that may not be evident at first. They may have heart, eye and hip problems as well as metabolic problems that may not show up until they reach adulthood (diabetes, thyroid and endocrine and kidney) problems. Because the breeding was done without regard to health, puppy mill dogs tend to be less healthy then dogs that are bred by reputable breeders. So if you are willing to take on the possibility of medical problems, not just now, but later in your puppy's life, I would go forth with the adoption. It doesn't mean that the puppy WILL have problems, its just a risk that is taken. I have a rescue puppy mill dog and I know that he may have problems and I am willing and able to take on those problems as they occur, if they occur. He is the best dog though, and I love him without condition.
Good luck! Shih tzus are wonderful!
Additional Info added: Again I will reiterate, the medical issues may not be apparent now when they are little. They may show up in adulthood. So if you are willing to take on that issue then go for it. Ok, rescue is taking the pups before they make it to the pet store so pet store and puppy mill is not profiting. You are not contibuting to the puppy mill situation because you are just paying for covered expenses. Puppy mills sell to pet stores, inferior, poorly bred animals that are sometimes very sick and very over vaccinated and the pet stores sell to public for exorbenent prices. THAT's what contributes to the profiting of a puppy mill. Not you purchasing for a minimal amount from a rescue agency. What I suggest is you go look as many Shih tzu breeder sites as you can and look up what each breeder breeds for...most are temperment and health, and look for the puppy that closest fits that match. Also go to the AKC site and see what the standard for that breed is. If you pick a pup, bring it to the vet as soon as you can, for an independent opinion. Make sure your contract with the rescue group has a plan for sick or genetic issues if your vet finds any, so you will be protected and not stuck paying high vet bills right from the start.
I hope that better clarifies the situation.
Owner of dogs for 18 years