I buy mine at a large grocery store. Not every store carries it, but if you have a large chain grocery store near your house, chances are they might have some. In my experience, it is kept in a special freezer near the check-out lanes, not in the regular freezer aisles accessible to customers. I always have to ask someone to bring me dry ice. It is usually sold by weight, not by volume. For this recipe, I used 5 lbs of dry ice over the course of a 3-hour party, but most of that was because I wanted to keep the smoke going consistently. 1-2 lbs is probably fine for most uses. Do not buy dry ice more than an hour or two before you need it, because it will evaporate. It's best purchased shortly before you will use it. Store it in a cooler with the lid cracked so the gas can't build up.