any grocery store or hardware store should carry ammonia. you want to make sure that it is 100% ammonia with no additives (dyes or perfumes). the best way to tell if it is 100% is to shake the bottle. if it bubbles it is not pure.
Go to Wal-Mart, they have it. It's called Clear Ammonia, it's located at the cleaning aisle and it's really cheap, cheaper than fish cycling three to four time, it's only 97 cents. Hehehe ...