I make my own by placing a piece of blue painter's tape on the bed of my machine. I raise the presser foot, lower the needle, place a ruler up against the needle, lower the presser foot, make sure the ruler is running straight from front to back, and draw a line on the tape with a Sharpie marker. When sewing a square on, I just line up the points with the drawn line. The trick is keeping the foremost point on the drawn line while sewing.