John o'Dreams by Bill Caddick uses a tune from Tchaikovsky's Pathetique Symphony. I imagine that's Public Domain now, even in America.

I never ask where a tune comes from when I'm writing a song - it just floats to the surface. Likely as not, some time later I'll decide it's pretty well the same as the tune of some song I've heard. Then I tell someone else that, and they likely tell me "No, it's not", because I'm not that accurate at remembering tunes I've heard.

Songs are there to be used to make new songs. It's not stealing, because the old songs are still there.