Do NOT buy a puppy unless you see the OFA results for DM testing for that pup- either the parents should be tested resulting in it being impossible for the puppy to be at risk for DM, or the puppy should be tested.
Safe breedings are: At Risk, Carrier, or clear in one parent to clear in the other
No other breedings are guaranteed to have puppies that will not get DM, so don't believe the breeder who claims she has never had DM in her lines. If you cannot go to the OFA website and see for yourself her test results, do NOT buy a puppy from her. You are better off with a backyard bred pup as DM is much more prevalent in show lines.
There are some responsible and caring Pembroke Welsh corgi breeders, and I can help you to find one. These breeders are testing their breeding stock for DM and attempting to lower its incidence. The majority are NOT TESTING. To quote one breeder, "There are enough pets out there." (As in, it doesn't matter of a few get DM.) To quote another, "DM is a disease of geriatric dogs." (Geriatric at 8 years old?) These are direct quotes from corgi breeders, who will try to tell you they are doing health testing. My friends, since July 2008, health testing in dogs includes testing for DM, and if they aren't doing it, you could end up with a dog that slowly and irreversibly goes down, becomes paralyzed, and dies of this disease.