Unlike some "dollar" stores, everything in Dollar Tree is priced at $1 or less (candy bars, for instance, might be 3/$1). That's the reason Dollar Tree would be my first place to shop if I were going to decorate for a football party. Their stock changes weekly, if not daily, so I never know what I'll find within their doors. Even if it wasn't football themed, they sell lots of solid color plates, napkins and table covers that would be suitable for a football party. Last year, they had banners and flags in a football theme.
Dollar Tree also has an aisle filled with snack foods. They often have pretzels in stock which are always a good snack food.