I have a recipe for homemade spray starch that I recently ended up trying, as I ran out and didn't want to run to the store:
Homemade spray starch recipe
This is a great money saver and it does an excellent job.
Dissolve 1 teaspoon to 1 tablespoon cornstarch (depending on amount of stiffness you want) in a few tablespoons of cold water in a heat proof 2-cup measuring container. Add boiling water to make one cup, stirring constantly. Then add cold water to the 2 cup line. Let cool and use in a spray bottle.
Shake it every time you spray.
You may have to dilute it a little if it is too thick or builds up white flakes.
Discared after a week or so and make a new batch. You can also refrigerate between using to keep it fresher.