Almond flour is used in baking as a substitute for grains. Almond flour should be refrigerated and then it will last six months.
You do not have to sift almond flour but it does compact. The amount that you use can vary considerably depending on how hard it is packed down so note what you do.
Almond flour is sold at Whole Foods in one pound packages. Very fine quality almond flour is sold in five pound bags through Lucy's Kitchen Shop. http://lucyskitchenshop.com. See Cookbooks and Websites.