From Nookipedia, the Animal Crossing wiki
Roman Candles are a type of handheld object given to the player by Tortimer during special events (such as the Fireworks Show) in the town. They are similar in appearance to the sparkler, but unlike the sparkler, it glows a certain color when activated and shoots out sparkles in the direction the player is pointing.
To get more Roman Candles, the player can drop the ones already acquired and then ask for more from Tortimer. Using this, the player can fill the whole inventory with Roman Candles up to two times. After that, Tortimer will say, "SORRY! Must have run out of fireworks! Maybe next time!", but he might also say other things to the player too.