This time of year they really aren't on display.  But if you ask around at the small shops in your area, they should be able to point you in the right direction.  Of course, if you go looking around the Chinese New Year, you will find more fireworks than a pile of pyros can dream of.

I think it's similar to back home. There are certain times of the year where they are openly sold, and you just have to stock up. In America, they're really only available around the 4th of July. In China, mainly out around New Year's.

there are small stalls near temples and / or large pagodas.  on some, besides regular offerings, they do sell firecrackers and sparklers year around.   or as already suggested, go to an open market and ask questions, you will be pointed in the right direction.