I bought my Kobo-branded Sleepcover (says so right on the back of the Sleepcover) at the Chapters store in Mississauga, Ontario, Canada. It most certainly does have an elastic strap to keep the cover closed. It's the same covers that are for sale on Indigo.ca (Chapters and Indigo are the same company); I have the one in heather grey, and like it very much:
http://www.chapters.indigo.ca/ereading/accessories/Glo-Fabric-SleepCover-Heather-Grey/681495000296-item.html