Fake teeth mouthpieces are a great addition to a costume that requires the character you are dressing up as to have specific types of teeth to really make their outfit have a realistic punch. These fake teeth vary in price but can usually be bought very cheaply, depending on how realistic they need to look.
Most costume and eclectic toy stores have a wide variety of fake teeth that work with many different outfits. Most of these cheaper teeth simply fit into the mouth over the top and bottom teeth, not really requiring any augmentation. Some of the more expensive mouthpieces from these stores can be shaped to fit your teeth like an athletic mouth guard--dropping the plastic into a pot of hot or boiling water, softening it, removing it, biting down and letting your teeth imprints take hold as the plastic dries. These mouthpieces are definitely better and make it easier to talk while wearing your costume, as they won't fall out.
Many different types and styles of fake teeth that are specific to the costume or outfit you have in mind may not be available at the costume store you are visiting, which may confound your costume coming together properly. For this, try searching eBay or some other online marketplace to locate the right type of teeth for your particular costume. There will be much more selection on a site that reflects people selling costume parts they may have used once and now just have sitting around. Just make sure you sterilize these teeth once you have bought them, by soaking them in pure grain rubbing alcohol.
Custom Made Fake Teeth
If you have some money to spend or want your fake teeth to look as close to real as possible, custom costumers and designers can create teeth for you out of high-quality, dental grade acrylic. These teeth and fangs fit normally over your teeth and gums so you can still talk normally while you wear them. They also have a layer of hydroplastic which can have their shapes changed numerous times, so they can fit any type of mouth or many different types of mouths.