Another idea, would be to use a mandoline. I am not sure how this is done, but mandolines often come with instructions on how to cut fries. A good one can be purchased for around $90.00. When making the fries, you also may want to consider a kevlar glove to protect from mishaps.
A real french fry cutter, on the other hand, can get expensive. They are heavy, and are likely to only cut one size. Expect to pay roughly $300.00 for one of those (new).