Doesn't matter if the vodka is potato or not. The starch in removed in the process of making the vodka. Use the cheapest vodka.
I had a lot of stiffener for crochet items. I diluted and used it as a heavy starch. Worked fine. I always use a pressing sheet when using Terial or other stiffeners. Theses sort of sprays will scorch and build up on the iron.
I bought Terial from HSN some time ago when it was on sale. HSN usually doesn't have anything on a sale price lower then their price unless it will not be in stock anymore.