Can I second what gryffnn said? The only time this ever came up for me, I emailed the creators of the bits I wanted to use - and they all said 'sure go ahead, we don't mind!' Though I was quite happy to pay if they wished. Just goes to show what a little courtesy can get you. (it turns out my attempt didn't bear fruit so I never got paid anyway. sighs, oh well... But the thought was there and I'm still grateful for the contributors' kindness)