DIY
Sew 3 edges of your pillow together, leaving the bottom seam open. Now take one piece of your fabric and press the seam allowance up onto the wrong side of the fabric (or to the inside of your pillow). Pin your zipper in place.
Line up your pinned zipper and fabric with the zipper OPEN. Stitch the length of your first pillow edge to hold the zipper in place. Prepare your opposite piece of fabric in the same way, with the pressed seam allowance.
Zip up the zipper. Take your second piece of fabric and butt the pressed seam up against the first piece of fabric already attached to the zipper. Pin in place.