You can buy them on the web, but home depot sells the clear plexiglass sheets that the thing is made from, so you can just get a fine point sharpie and a ruler and make your own…
Glue one of THESE to the bottom of your clear acrylic. Measure the exact same distance from each end to a point about one foot away from the ruler to establish a perpendicular. Scratch the perpendicular in to the plastic and mark with a Sharpie. Wipe with some alcohol and it’ll leave a permanent line to reference your stringer or centerline.