You should find no shortage of store-bought "heroes uniforms" at party stores and online (try www.costumeexpress.com and www.anniescostumes.com). But if you want to go the homegrown route, the right accessories can turn a solid-colored sweat suit (black for police or red for firefighter) into a hero-inspired uniform.
For the police officer, add a badge, a police hat, and a pair of handcuffs. The fireman's must-have props: A fire hat and a badge. Turn your little one into a health professional with green scrubs or white pants or a white skirt and white top, and a white jacket as a "lab coat." Add a toy stethoscope, thermometer, and doctor's bag or nurse's cap, and your junior medic is ready to save lives.