I've heard some conflicting reports regarding the exact instruments. My ears tell me plainly they aren't flutes, but instead sound like Irish Low Whistles -- a flute-like instrument that uses a recorder-like mouthpiece. Hobgoblin Music must have at least two dozen varieties, and there are several YouTube videos showing how to fashion them from various materials. One could go through a lengthy experimentation process finding some that best emulate the tonality of those used on the recording. I have a hunch that the whistles were taped for the Mellotron and played live that way, but I can't say anything about the actual studio tracks. For those like me, who wish to find a set of those for ourselves, they come in differing ranges & sizes, each one imparting a somewhat distinct voice. The trick would be to find those that most closely emulate the ones heard on the original, while somehow avoiding the need to have to purchase fifty of them in order to experiment. I don't believe there's any need to look for a different instrument. A Mellotron can play a recorded instrument, starting with the actual capture of the instrument's initial attack envelope, and sustain it for up to eight seconds, the length of tape controlled by each key. The whistles played on Stairway to Heaven seem to need only sustain for about four seconds before a different note is sounded. If someone were wishing to sample those sounds for their own performance uses, keep in mind that you can use studio processing to enhance the inherent character of an instrument, but that little can be done to impart a natural-sounding character to an instrument that it doesn't possess on its own. If you want to give a sound an artificial character, however, the sky's the limit. If I were tasked with re-creating the exact sound of the whistles on the original, I'd start with the largest Low Whistles that would cover the musical range expressed on the original, and proceed to get a good capture that requires the least processing, hopefully needing none. This is an adventure I wish to follow through with someday.