Its called "Batting Tape" You can find it at Nancy's Notions for one place and I'm sure other online sites have it too. It comes in a couple different widths too so I order the 1.5 x 100 yds size.
I keep the larger pieces and attach them to others close to the same size. I keep a paper with what the size is afterwards pinned to it so I know the size before I add another section to it. Any small pieces that are at least 8 x 8 and larger I keep for embroidered quilted blocks or for adding to pillow covers I make. I also use it for when I'm reunholstering chairs or embroidered items for picture frames.