Well, i got mine at the grocery store. You could also try hardware stores, etc. Anywhere they sell cleaning products, and it will probably be a generic store brand. If someone asks what it's for, don't tell them you want it for your fishtank, they'll either give you a really weird look or think you're lying and you're cooking drugs with it or something. You can tell them your grandma sent you out for some ammonia that she uses for cleaning, she doesn't like any of that new fangled stuff hehe. The kind you need should be marked "pure" or "clear" ammonia, should not have any color to it (no purple dyes, that sort of thing), nor fragrance (lemon is common), nor detergents. If it has any of those things it should be marked on the container, but you can shake it to be sure. The kind you don't want will foam up really strong if you shake it, the kind you do want will be basically colorless (maybe slightly yellowish) and may have a few bubbles but won't foam up when you shake it. It should be really cheap, i think my bottle cost me 89 cents US.