Hey! I can't seem to find the original song, but if you don't mind english lyrics try Animeja. Search animeja and in her album there is a version of it that I like. :) I think that the music box version is cool too though.

this is a ripoff off the orginal song itunes, jusut get us the original song!

Well, for now, I guess for now this is the closest thing to Arrietty's Song you'll get. But I would prefer the original Arrietty's Song single to be on here. So I'm not buying this; nice try iTunes.