I buy most of my gluten-free flours from online sources. I can often get my flours through Amazon.com. A big plus is you can buy almost all the flours used in my cookbook through Amazon.
And www.vitacost.com is another site with a great gluten-free selection. They've actually sometimes beat amazon as far as pricing on gf flours. New to Vitacost - use this link and save $10 off your first order.
Another company I particularly like is www.iherb.com. I've been shopping with them for years now. Low prices, super fast shipping and terrific customer service. And if you're a new iherb customer, enter the coupon code CEL291 when checking out and you'll receive $10 off your 1st order of $40 or more.
*Brown Rice Flour: Based on the recent findings of high levels of arsenic in brown rice, I'm now just making my flour mix with white rice flour and substituting any brown rice flour in the mix with white rice flour. Earlier printings of the cookbook may have brown rice flour included in the mix.
RECENT UPDATE: White Bean Flour: If you're using Authentic Foods Brands white rice flour and sweet rice flour, I've recently discovered that you do not need to add any bean flour at all when preparing my gluten-free flour mix. Authentic Foods Brands flours are milled so finely it's not necessary. So that makes my gluten-free flour mix even easier. Just two flours to mix together and you're all set to go!
I no longer use any gums in my gf baking. Konjac powder is the best xanthan gum substitute.