Supermarkets are the main place U.S. consumers buy groceries, but dollar stores are gaining momentum as are drug and convenience stores, researchers say. Perception Research Services International said their annual shopper research survey found 91 percent of those surveyed said they purchased groceries in the past three months at a supermarket, down from 92 percent last year. Mass merchandisers were the supermarket\'s largest competitive threat, with 73 percent purchasing groceries there in the last three months, the survey suggested. The survey found 35 percent said they shopped for food in a dollar stores in the last three months, compared to 32 percent in 2011, 47 percent said they shopped in a drug store for food in the last three months, compared to 46 percent last year and and 24 percent shopped in a convenience store for food, compared to 23 percent last year. Consumers purchased beverages and food generally at the same rate across mass merchandisers and dollar stores, but cleaning supplies and personal care items were purchased more often at dollar stores. This study was conducted among 1,500 U.S. shoppers age 18 and older last June. No margin of error was provided. Food advocates often warn consumers to read the labels of food sold in dollar stores because many food items come from unregulated countries, which might not have the same quality standards as U.S. name brands and while they might not be expired, they might only have days left before expiration.