Fabricland do decent cushion pads, much cheaper than Merrick & Day - not quite such good quality, but mine are still comfy after several years!
When making a lot of cushions, I bought a zipper roll so I could cut exactly what I needed, and found it so much easier than fiddling to put a zip in. As I seem to need to wash mine fairly often (!) I don't really like sewing them up by hand.
I cut a square of the 'nice' fabric to the exact size of the pad (so the seam allowance then makes it a bit overstuffed). From the plain fabric I would cut the same size but 2 cms. longer one way - cut this in half and sew the zipper on to join them, with a 1cm allowance. Zip up, now 2 squares the same size that can be sewn together.