potato- have you ever tried steel's gourmet anything? they are awfully pricey, but if the texture's right, as it *claims* to be, perhaps it would be worth it.
i have heard about the notStarch, but i have been offset about the price (netrition has it in a huge quantity for a huge price) because i had not known anyone who could testify for sure about its effectiveness. does it have to be cooked to work, or could it thicken, for instance, a salad dressing?