Hi Sharon,
I think I found it at Hancock's, or maybe JoAnn's. Prior to that, I used a fine Sharpie and drew my own lines, which work okay, but isn't perfect.
I also have a foamcore board with just plain flannel on it that I use probably more than the bigger one with the grids. Sometimes the grids detract from what I am trying to see. However, it is great for getting rows straight, etc.
Hope this helps...
:)
Anita