This instructable shows you how to make your own with just a computer, printer, paper and transparencies.
I'm assuming that you have the Gimp image editor installed. Hopefully you can translate this as needed for other programs.
Step 1: Create the base image and gridlines
Create a new layer called 'Grid 1'
Select that layer from the layer dialog
Create the grid by selecting the 'Filter' menu then 'Render' -> 'Pattern' -> 'Grid'
unlock the horizontal and vertical settings for width, spacing and offsets.
- Width 0,18,0
repeat for layers 'Grid 2' to 'Grid 7' using horizontal offsets of 3,6,9,12,15 and18 respectively.