Your best chance to find a purebred mini schnauzer is from a reputable breeder. You have to watch out for puppy mills and BYBs. I believe someone above has posted a link to a thread about all of the things you need to look for when searching for breeders. However, the lower priced the dog is, the more likely it is that the breeder is a BYB or a puppy mill.
just an example of a reputable mini schnauzer breeder: Champion Miniature Schnauzer | Mini Schnauzer Breeders | Texas
The website also gives good advice in the puppy section about what to look for as far as finding a good breeder goes.
You will not find a mini schnauzer puppy from a reputable breeder that is really much less than 1200-1500 dollars. Anything less means that the breeder you're looking at is probably a BYB, puppy mill, or just an inexperienced breeder. It also means that your puppy could come with health problems (some fatal), behavioral problems, and more.
Your best bet is to look on petfinder.com (but I would see if you can meet the dog first before you buy) or go to a shelter. I don't think I have ever seen a mini schnauzer at a shelter, and if there are any, they aren't going to be there for very long (as people are always looking for mini schnauzers cheap).
Be open to other breeds with similar traits. I would just go to a shelter and look for a dog that would fit your lifestyle. Don't go just on looks, either! I understand that you have your mind set on this one breed but there are lots of other dogs out there
Also, my problem with the breeder you have showed us is that they only have (I think this is what it said) 3 dogs that they breed. What? A good breeder would have many more dogs. And think of the amount of times these dogs have been bred? This breeder is obviously inexperienced.