I use Niagara bottle and mix it to the stiffness I want. It is available at supermarkets. I think most of them carry it or other brands. Look in the isle where laundry stuff is. If it isn't available at your local store, talk to the manager and ask them to carry it.
I like the bottle starch rather than spray cans because less garbage ends up in landfills. If I could find powdered starch, I would use that.