I found it! I remember feverishly putting it together one night after homework and dinner to wear the next day in 8th grade (mid-1980s, kiddos!). I dug through the multi-generational button box for a variety of metal buttons and found a scrap of black elastic - about 3/4" wide. Then sewed them all on, sitting in the Lazy Boy recliner. 
Some of the buttons are really cool. Too bad the elastic is shot! Originally you couldn’t see much of it between the buttons.