In most regions of the U.S. it can be difficult finding these at stores. That's unfortunate, because the antioxidant content in black raspberries vs. red raspberries is a difference of night and day.
It is true that the red variety are still quite nutritious and actually have more antioxidants than most fruits and vegetables. But their ORAC value is around 1/4 of what the darker variety has. Although it can be difficult finding them fresh, many supermarkets do carry them frozen which are just as nutritious for you.
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010