Someone on the Bikejournal forum (Kin perhaps?) came up with a recipe for a gu-like substitute he calls Rocket Fuel. It uses brown rice syrup & agave nectar, which I gather are both available at health food stores.
I haven't tried it yet, but here's the basic recipe:
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Kin's Rocket Fuel
3 parts Brown Rice Syrup (50% complex carbs, 40% maltose, 5% glucose, 5% water) - $3.50 for 1 lb. 5 oz.
1 part Agave Nectar (95% fructose, 5% water) - $2.50 for 11.75 oz.
salt
vanilla
a little water to thin it up
After putting each of the ingredients in a plastic GU dispensor bottle in the proper proportions, dunk it in very hot water to help it mix better.