HOLD ON...are you SURE it's potato FLOUR your want, and not potato STARCH Flour? there's a big difference, and i highly doubt a pound cake recipe would call for potato flour (a potato bread recipe perhaps, but not a pound cake). potato starch, on the other hand, is used to produce a more tender crumb in certain baked goods.
potato flour is made from whole potatoes that have been cooked, dried & ground into a flour, while potato STARCH flour is the isolated starch from ground raw potatoes.
please don't try to use the flour in place of the starch, it'll ruin your cake. double-check the recipe before you go looking to buy the ingredients.
if it's potato STARCH you're looking for, you can find it in the baking ingredients section at WFM along with all the other bagged flours & grains from Bob's Red Mill.