If they have it, they normally keep it in the fridge so you won´t see it. 75% of the vedulurias in San Telmo have it.
You can also special order just about ANYTHiNG if you give them two days notice. I once saw someone order unbrined olives to brine at home--didn´t even kmow that was possible! Any spice, weird vegetable, exotic fruit, nuts. . . they often don´t buy them because there is not enough market, but they do exist.
They of course will only order if you are a customer who has gone there for a bit.