Are you saying you want to make one?
Firstly, the waistband has to start out at the largest required size.
I would not have the buttonholes in the elastic, but in the fabric at the ends, with the elastic always hidden inside the back waistband. Then all that is visible is the button/ buttons on each side. I would make a normal waistband, but have the outer fabric in three parts with the centre section overlapping the side ones. It must not be too stiffened to gather up slightly when pulled in. Stitch down the ends of the side sections to the backing so they do not catch on the moving sections. perhaps line the middle section so that there are no ends to catch there either. For the middle, make interfaced strips of the main fabric, slightly narrower than the main waistband and tapered at the buttonhole ends, long enough to be hidden when on the tighter setting, sew them on to a length of elastic of a matching width. Thread through the 'tunnel'.
I hope this makes sense. Much easier to make than to describe!