@Al Rassasi Fire Works on Madina street will have fireworks, as for an American Flag...hmmmm, my friend was looking for one for a while and didn't find one..I would say check out @Downtown . But it will be harder now since the US is out of the World Cup
Wust Al Balad - If you don't mind to buy them second hand, there are a few shops in the Balad that sometimes sell them. One is close to Al Quds (the one on the way to Ras al Ain) @Downtown
@Virgin Megastore is cheap, but treat it with care or else it will probably break..Not the best quality. I've seen some @Downtown too, but just make sure you check the quality yourself before you buy!