We make our own too. Commercial ones usually have quite a lot of sugar in them which I'm not a big fan of - seems to defeat the main purpose. Home made ones also have heaps more good bacteria in them than store bought. You can get yogurt makers for $20 from Woolies, and either use the packets or starter culture. It's really easy.
I used to do that, you can also get electronic ones (they just keep the milk warm though) that use just milk, milk powder (if you want) and some yoghurt. You then just save some of your first batch to make the next one. They make a lot of yoghurt though, mine made 1.5L at a time. But, like Yeddi said, you know exactly what is in it cause you put it there yourself. Why I make my own bread to.