I don't know if anyone else had a problem finding these things, but they aren't too hard to acquire. The Volcano 4000 firework is located in certain lockers; they appear more often after all missions are beaten.

As for the water balloons, they will be unlocked once you help the guy on top of the building next to the bike shop in Bullworth Town. Jump the fence in the back and climb the ladder to get them. From then on you may fill balloons at any source of water (water pipes, drinking fountains, etc.).