You have me stumped, Nik. The one folk song I know of that Copland used was the Shaker hymn Simple Gifts. I heard recently that Copland used the fiddle tune Bonaparte's Retreat in a symphonic piece, and I'm frustrated because I don't know which Copland piece has that song. Can anybody enlighten us on these two pieces?

Another folk song used in a symphonic piece was the spiritual Goin' Home used by Dvorak in the "New World" Symphony. Ralph Vaughan Williams used folk songs in many of his compositions, most notably his "Greensleeves" suite. I think he also did a "Folk Song Suite," but I can't think of any of the songs in that piece.
-Joe Offer-

P.S. Note the spelling of Copland's name. I didn't know the spelling until I looked it up just now.