You COULD buy cider vinegar in stores, but you are not always getting a quality product.  It is fairly easy to make this at home, and you can be sure it is pure.  You can also keep the mother alive and make new batches whenever you need it, and it only gets better with age. You will need: a non-metallic container with a relatively tight-fitting lid (i.e. lidded casserole dish or crock pot) apple peel and cores (this works best with about 12 apples' worth) pure water (NOT tap water) plastic wrap (optional) apple juice (optional) Note:  If you're going to keep making vinegar, don't wash the crock with soap or detergent between batches. A rinse and wipe with a lint-free paper towel is enough.

Make sure the peels are clean and free of any wax or pesticides.  Put them in the container and just cover with the water.  If you don't have 12 apples worth, you may want to add a little apple juice to help with the fullness and fermentation.  Every day or two, stir the mix to aerate it, and skim off any foam that forms from the fermentation.  As the vinegar ages, the flavour will develop and the liquid will become less cloudy.  If you like, you can cover the top of the container with plastic wrap, and then put the lid on.  This keeps it pure and ensures that those pesky fruit flies can't take a dip. Taste it occasionally to see how it is coming along.  When the liquid is to your liking, strain and bottle.  Keep some in the container and occasionally feed it peels and water.  This is a vinegar mother, and will help you to make future batches.

This can be used in cooking, for cleaning (excellent on glass) or taken as a tonic by diluting 1-2 tsp. in water twice a day.  If you're thinking of taking cider vinegar for weight loss, THIS is the best way to take it (not the store-bought or pharmacy stuff).