The Japanese love fireworks, especially in the hot summer. The fireworks season starts in June and ends in September. Everywhere you look you see fireworks. It could be posters about shows and festivals, advertorial (even by big companies), or real fireworks in the shop window.
Big Japanese companies like Yahoo!, Kadokawa's Walker+ and Suntory Beer and JR East have a special website for the fireworks. Even a magazine exists with all firework festivals, shows, tips and more. -Yes, I did bought this magazine!
Consumer fireworks are available in all kind of shops. I found fireworks at Don Quijote Shibuya (Tokyo), Loft at Niigata and a few 7-Eleven shops in Nagaoka. Most fireworks are really small, like sparkles, morning glory sparkles, ground bloom flowers, small rockets (moon traveler bottle rockets) and roman candles. The bit bigger ones are single shots and fountains. You can buy fireworks if you are over 20 years old. Use the fireworks at parks, beaches, or at firework festivals where you don't annoy people.