Find the center point on a square of paper by bending it in half and pressing a crease at the midpoint. (Don't make a sharp fold along the entire length.) Open the square, rotate 90 degrees, and repeat.
You can easily make your own with a canvas, a paint brush, masking tape, and three colors + white acrylic paint. Just tape up your canvas every which way, paint a gradient of each color in the taped segments and remove the tape