I'm not sure about this site but Arm and Hammer is good for baking soda and I've always found Jewel Mills Flour to be great. Rarely even needs sifting.
LOL... Seriously, we don't give out "best source" answers because all sources are approved. We don't put one against the other. Write to a few and go with the one that you think will provide you with good service. Some will even do samples if you ask.