If you google "Munsell Neutral Shade Chip" you will find some places to order it from.
If you are just looking for examples of neutral shades then go to Home Depot and look for the neutral gray sample cards for Glidden paints. Any chip with a code like "00NN 72/000" will be a neutral shade of gray. The 72 indicates how light a shade it is. All the other numbers should be 0 and the two leters muxt be NN. The color names to look for are Universal Gray and Snowfield. These are color sample chips so they are not made to the same standards as the Munsell chips, but they will give you a reasonable visual reference for what a neutral shade looks like.