I went to Jo-Ann's and purchased an interfacing with a 1" grid printed on it. If you order it online you have to buy a whole bolt, but in the store it is sold by the yard. The blue grid lines printed on it were not quite dark enough for the purpose I needed, so I drew over them with a fine black Pigma Micron pen. Just be sure to put some type of paper under the interfacing to absorb what bleeds through.
Here is what it looked like after I had drawn over several rows. It will give you an idea what the finished product looked like.