Nopales, the leaves or paddles of the Opuntia cactus are also used in Mexican cooking. You can buy jars of nopalitas from either of the Mexican stockists I mention in this blog but like boiled okra, they can be rather slimy in texture.
If you want to avoid gloopy cactus, a much better method of cooking is grilling/baking the paddles. Prepare the cactus by removing the spines and cut into bite-size cubes/strips. Toss in olive oil and roast on a baking tray for 20-30 mins. Serve with a squeeze of lime and coarse sea salt.