It's in the elastic section, in packages. It's on sale this weekend 50% off, but it's cheap anyway. You might also see my post on the board right now about using the "elastic panels" (don't know what else to call it) that's on clearance right now. VERY easy and any machine can do it since you just use your straight stitch. I mean, regular shirring just uses a straight stitch too, but my machine doesn't like it.
ETA: it's white.