You can make your own by mixing wheat bran, wheat germ and all purpose flour according to this recipe.
For sifted all-purpose white flour, wheat bran, and wheat germ having densities of 125, 50, and 80 grams/cup, respectively, one cup of graham flour is approximately equivalent to 84 g (~2/3 cup) white flour, 15 g (slightly less than 1/3 cup) wheat bran, and 2.5 g (1.5 teaspoons) wheat germ.
Regular AP flour is made from the endosperm of wheat kernels. Graham flour is a special type of whole wheat flour. Traditionally, I read that the endosperm would be grounded separately from the bran and germ. However, nowadays it seemed that the distinction is a bit fuzzy. You can use graham and whole wheat flour pretty much interchangeably.
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For sifted all-purpose white flour, wheat bran, and wheat germ having densities of 125, 50, and 80 grams/cup, respectively, one cup of graham flour is approximately equivalent to 84 g (~2/3 cup) white flour, 15 g (slightly less than 1/3 cup) wheat bran, and 2.5 g (1.5 teaspoons) wheat germ.
Source: http://en.wikibooks.org/wiki/Cookbook:Graham_Flour
I made my own and made graham crackers with it using the Cook's Illustrated recipe. We made scratch marshmallows to go with them. They were awesome!