I haven't found any blend that is a perfect substitute for wheat flour, but this one is very versatile. The flours and potato starch are all Bob's Red Mill (available at most local grocery stores; and certainly at the health food stores.) I'll warn you now, the xanthan gum is very expensive, so be careful of sticker shock. You can find it at health food stores, or it is also available online from Bob's.
1 cup white rice flour
1 cup brown rice flour
3/4 cup tapioca flour
3/4 cup potato starch (that's "starch" not flour)
4 teaspoons xanthan gum (you'll find it at health food stores)
Just dump all ingredients into a quality, airtight Tupperware container, seal, and shake to mix well. I usually make a double batch at a time.