When choosing a miniature schnauzer breeder, be sure to take your time. Look at a lot of breeders, and consider references from the AKC.
Do your research.
Remember no puppy mills, and no backyard breeders.
Two things (out of about 50) to look for in a mini schnauzer breeder:
1. A three-to-five year health guarantee against serious genetic defects. (I’d run away from one-year guarantees, it can take three to five years for some miniature schnauzer genetic disorders to present themselves.)
2. The breeder is willing to take back the dog for any reason during its lifetime. (A sign that the breeder cares about their dogs and not just the money they get for them)
Here are some good breeder criteria, though not miniature schnauzer specific.
For your viewing pleasure: Flickr Pool of Mini Schnauzers