Russ, I would try an artist supply store or a blueprint/engineer's supply house. There is also grid paper that is easy to find, it comes in different size grids, quarter inch, half inch, but there's no numbers along the edges. Googling 'grid paper' gives lots of hits, here's on-line grid paper:
Carolyn, it's just one I had in my file, so I don't know the variety. I'm after the grid array.
I've seen similarly gridded photos where the fruit and leaves are arrayed on the same grid; and I notice the color compensation dots (sometimes they are squares), so I assumed it is a standard presentation of some sort. And I've seen it used for other plants.
I've checked a couple of local art and engineering graphics suppliers locally, with no luck.
Something like this.
I don't have a handy place to post this. I can email it in most image formats plus postscript and pdf formats. I haven't printed it so I may have to tweak it to get the sizes perfect. It is set up to be printed on 8.5 x 14 inch paper