We order reagent grade from Fisher Scientific and mix it ourselves.  We use it mostly for 5% nital solutions for metallography and 20% for macroetching.  Be safe - Always pour acid into solvent (ethanol or distilled water).  http://www.fishersci.com/wps/portal/PRODUCTDETAIL?prodcutdetail=%27prod%27&productId=4843388&catalogId=29104&matchedCatNo=RABN001025D||RABN00104X2||RABN0010500||RABN00106X2&pos=3&catCode=RE_SC&endecaSearchQuery=%23store%3DScientific%23N%3D0%23rpp%3D15&fromCat=yes&keepSessionSearchOutPut=true&fromSearch=Y&searchKey=nitric||acids||reagent||reagents||acid&highlightProductsItemsFlag=Y

There is also another place we use for other etchants - Redbird Service (http://www.redbirdservice.com/catalog23/solutions%20n.htm).  You can buy it pre-mixed to 20% or whatever concentration you want.