The basic lead sheet for it is in the C and Eb Real Books (legal versions) but with 'ambiguous' chord symbols, understandable given the harmonies. FreeHand has a nice and inexpensive sheet here:
Icarus Sheet Music - Oregon - Ralph Towner - 403955 | FreeHand Music . Ralph's official version is available in volume 1 of his solo works - both volumes are available cheap online, and highly recommended. Hope this helps!