Starting Monday, consumers will be able to buy hearing aids directly off store shelves and at dramatically lower prices as a 2017 federal law finally takes effect. Where for decades it cost thousands of dollars to get a device that could be purchased only with a prescription from an audiologist or other hearing professional, now a new category of over-the-counter aids are selling for hundreds of dollars. Walmart says it will sell a hearing aid for as little as $199. The over-the-counter aids are intended for adults with mild to moderate hearing loss — a market of tens of millions of people, many of whom have until now avoided getting help because devices were so expensive. "From a conceptual point of view, this is huge that this is finally happening," said Dr. Frank Lin, director of the Cochlear Center for Hearing and Public Health at Johns Hopkins University in Baltimore. He predicts it could take a couple of years for the new market to shake out as manufacturers and retailers get acc