Decide whether you need neem oil for a cosmetic or an insecticide. Obviously neem face cream isn't going to help your plants, and you don't want to spray a pesticide--neem-based or not--on your face.
Shop for the best deals among online retailers such as PlanetNatural or GreeNeem for pesticide use or Neem Products for cosmetics. You should also scout local stores. Most garden centers will either carry neem-based pesticides or be able to order them, and neem cosmetics can be found in almost any health food store and in some supermarkets that stock "natural" or "health" sections.
Check to make sure you're getting the right concentration. You can make your own insecticides sprays from pure neem oil; if you're buying a diluted product make sure that it is at least 70% neem. For cosmetic products, note whether neem is high or low in the list of ingredients. There is no rule for how much neem should be in cosmetics; you just need to make sure you're not paying a lot of money for "neem" on the label but not in the bottle.