Translation:I do not buy fruit in the supermarkets.
(American English) Singular can be used, especially if the phrase can be thought of like "I never buy fruit in the supermarket, I only buy it at the weekend Farmer's market." Basically if "supermarket" is being treated as a classification then singular would be correct (all of this type is forbidden). If instead it is referring to certain stores, but not all stores, then plural would be correct. "I never buy fruit at supermarkets a, b, and c."