I don't know where you could find it, but my advice is to forget about it anyway. Keep the arrow in your head. It's not that hard, and you'll learn it pretty quickly. Instead of counting "one-thousand-one, one-thousand-two" for your backcourt and closely guarded counts, try "red-arrow-one, red-arrow-two. . ." You won't need an extra whistle, or rubber band, or flap on your whistle.
Good luck!