.fla files of existing games


People are probably gonna think I’m pirating, but I had an idea the other day to practice my design skills, (not programming skills).
I want to take other people’s flash games, and modify them, in order to, practice game balancing. So change the speed of the player, amount of enemies etc.
I will after that never put the swf on the internet, neither with my name or that of the original creator on it. I’ll just delete it when I’m done.

my question is twofold:
what is the best decompiler?
Are there any sites that have complete .fla files with actionscript to download?

I’m not doing this to learn programming, just to practice game balancing.

I know that people are going to interpret this as piracy, and I don’t blame them.