Cactus Fruit in Tijuana?

Those are the fruit of the prickly pear cactus, known as "tunas." In Mexico, you often find vendors selling them in plastic bags on street corners. They are becoming more common in the US, especially at markets that cater toward immigrant communities from Mexico. Larger chain stores such as Tops, Giant Eagle, Stop and Shop, etc., will often have a basket of them in their specialty fruit areas. The websites listed will give you more background information and ideas.