Does anyone know where I can find the original short story Banshee? Which book/collection is it in?

According to Eller and Touponce's Ray Bradbury: The Life of Fiction (a great study of Ray's work which I highly recommend), Banshee first appeared in the September 1984 issue of Gallery, and was reprinted in Rod Serling's The Twilight Zone Magazine (October 1984); The Best of Gallery (Winter-Spring 1985); and in the U.K. magazine, Woman's Own (Mar. 15, 1986. It later appeared in The Toynbee Convector (1988) and in Bradbury Stories: 100 of His Most Celebrated Tales (2003). It may also have been interwoven into Green Shadows, White Whale (1992).
Hope this helps.
Best,
Ken

According to ISFDB, "Banshee" appears as chapter 27 of Green Shadows, White Whale, probably with a minor degree of rewriting.