Like Kelley said, pink slime is found in ground beef. It's the leftover bits and pieces of the cow with added ammonium. It's best to avoid buying hamburgers from fast food restaurants, frozen meat patties, and even some of the ground beef found in supermarkets. I would also question sime of the burgers from high end restaurants.
You can also ask the butcher to grind the beef in from of you. Most places will do that for you. Another option is buy your own meat grinder.