Stability of Chromic Acid I am told that chromic acid is exceedingly unstable and that it cannot be purchased as a pure reagent. 

Is that so? 

I don't find any mentions of stability or lack thereof on Wikipedia, and hey, if I wanted to, I could purchase it here:

http://www.grainger.com/product/LABCHEM-Chromic-Acid-8FV43

So what's the deal with having to make chromic acid in situ? Does this have to do with chromic acid's stability? Or something else? 

EDIT: oops I realized that the linked reagent for purchase is just chromic trioxide, which can be used to make 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.