Stability of Chromic Acid
"chromic acid" is also a nickname for chromic trioxide.  It is stable enough to be sold commercially.  Chromic acid in the strict sense ($\ce{H2CrO4}$) is not sold commercially, I think.  For most purposes what you're looking for is Cr(VI) plus acidity, so the usual practice of adding concentrated sulfuric acid to sodium or potassium dichromate (which have multiple uses and can be found in every lab) is preferable to having to buy another reagent.