I just assumed that every grocery store would carry those, but I haven't seen them, specifically. I've definitely seen them at
Cakewalk, a cake/candy/baking supply store on the South side. You might call first to check before hauling it down there, though.
Cakewalk
1741 W. 99th Street
Across from Cafe Luna just East of the Metra tracks
773-233-7335