Meat tenderizer refers to a powdered naturally derived enzyme powder. The enzyme most commonly used is papain which comes from papayas. Meat is sprinkled with the powder and the enzymes help to break down the meat fibers. The same result can be obtained by marinating meat in papaya or pineapple juice.
Sometimes the term meat tenderizer is used to describe a meat mallet which is alternately used to pound and flatten meat to make it more tender. These are readily available in any kitchen shop or even at your grocery or hardware store. You also just use the side of small plate or saucer which works pretty well too.
Purchase powdered meat tenderizer in most main stream grocery stores. A very well-known, reliable brand is Adolf's. You can also find it online at Amazon: Adolph's Meat Tenderizer
If you don't have meat tenderizer or just don't want to use it you can use one of these substitutes.
salt, dextrose, bromelain (tenderizer), and calcium silicate (added to make free flowing).