Definitely use campden, or freeze your cider if it is unpasterized. The amount of bacteria, spores and wild yeast growing on fruits would blow your mind! Thats why cider was originally made by just leaving it long enough until it self-fermented.
When using store bough stuff, check to make sure the only added ingredient is ascorbic acid (vitamin C). If they list anything else, it will likely contain sulphites which will prevent yeast growth the same way campden does.
I did the same thing as you last fall, bought a 19L jug of cider from the market and fermented it with an additional 2 kilograms of dextrose. Like Richard said, crush 1 campden tablet per gallon and let sit for 24 hours (NOt sure if the boiling is nessisary).