I think your best bet is to try and find a copy of "The Complete Lyrics of Ira Gershwin" (some libraries stock it). The song lyrics don't seem to show up in any of the usual Gershwin books - George had nothing to do with it as it was written specifically for the film. Walter Rimler gives good info (but no lyrics) on the song itself in his book "A Gershwin Companion". Also try music stores; since it was a published song it might just show up somewhere. Warner Bros. seems to have the rights to a lot of Gershwin material so that might be the place to start.