Buy the set, install them all, then sell the old extras?
Here's what I would do.
Go here www.sailrite.com
1 roll Seamstick tape p/n 129
1 piece Kal Glas p/n 5874
Get a piece of cardboard bigger than the window. Lay the window out carefully, and duct tape it down securely around the edges.
Cut out the old plastic window.
Put down strips of the adhesive tape around the perimeter.
Cut new plastic to fit to the outside of the tape line.
Carefully peel the tape backing and stick the plastic down.
Sew the edges, one stitch along the outside, one stitch along the inside edges of the tape.
Or - take it to any upholsterer/top shop and have them sew it. Should cost $20.
for under $70 a new window.