normally, one can't just "try on" insoles, one has to buy them and try them out... you could flex them to see if they have stiffness but the usual ones I find in stores are made to provide cushion and not stiffness...
so assuming that they are special in that way, I wouldn't even have a clue where to find "stiff" insoles... possibly from a pharmacy, certainly not shoe stores, or dept stores like walmart, target and the like...
now, it just so happened that I was discussing this with my friend over lunch and coincidentally, his doctor recommended a set of "powerstep" insoles and that they could be found on amazon...