I achieve 'heavy', 'light', or 'medium' starch by making my own. To make a medium starch, dilute 1 teaspoon of corn starch in a tablespoon or two of cool water in a 2 cup Pyrex measuring cup. Add 1 cup boiling water and stir well then add 1 cup cool water and stir well. Pour into a spritzer bottle and there you go! Two cups of medium starch.
For light starch, make it with less corn starch, for heavy starch, add up to a tablespoon.
Spray the front of your fabric and let it sit for a minute or two or longer. Turn fabric over and press. (You don't have to turn, I just do so starch doesn't build up on my iron (and any starch can do this, not just home made).
If you do make your own, remember this will only last about a week since there are no preservatives or additives (a good thing for you and the planet!). So if you have a bit left at the end of the week, just toss it out and rinse out the spritzer bottle until you want to make your next batch. Since this only cost pennies to make per batch, it's no problem throwing a bit out if you have to.