This is a tricky ethical question.
If you are trying to decompile a package of classes that you paid money for, and want to use the source to write your own stuff and resell it, then perhaps this is not such a good idea.
But if you are just satisfying curiosity, then go to
www.download.com and in the search box at the top right, search for '
java decompiler'
They have downloads for mp3 piracy software, why not java class files, right?
Morality is for the rich?