Find a large patch of wild cactus. If you do not have any on your property or know where to find the plants, call the local forest ranger service.
Harvest the spiny, purple fruits (also known as tunas) in September. Wear leather gloves and cut off the tunas with a sturdy fork and a long knife. Prickly pears are covered with many different types of spines so use caution.
Wash the tunas under cold water with a vegetable brush. Slit them open and put them in a pot. Add just enough water to cover them. Cook until very tender. Strain the seeds and pulp through cheese cloth. The juice from the tuna is often used to make jelly and prickly pear nectar. This is sold in supermarkets and natural food stores.
Extract cactus juice from the leaves (also known as pads) at home by manually crushing them.
Get cactus juice commercially for research and medication by exposing large quantities to heat or steam to soften the outer coats and extract the sap.