No, you can buy a tub of yogurt from the dairy aisle, however, you have to make sure that the yogurt is fresh and the culture is live. It will state clearly on the tub, "Made with Live Cultures" or something to that effect. Not all yogurt is made this way, so you will have to look for a tub that does.
Now just use a tablespoon or two of this live cultured yogurt as a homemade yogurt starter for your new batch. Each time you make a new batch of yogurt you can use a tablespoon of your last batch to start the next one.