ST. PAUL, Minn. --Retired furniture store owner Don Brock quit buying prescription drugs from Canada this year, now that he's signed up for the new federal Medicare drug benefit. The next time he needs a refill on Lipitor, his daily anti-cholesterol drug, Brock will go to a pharmacy near his home in Litchfield. The 74-year-old says he was saving about $300 annually buying Canadian; now, he figures he'll save about $500 buying through Medicare. Canada is losing traction as a source of cheaper prescription drugs for many Americans. Cross-border sales have fallen as much as 30 percent, according to the...