Crop failure is a very serious concern for any agricultural economy. Mitigation was made even more difficult without the fast communication of instruments like futures contracts or well-linked supply chains that modern agricultural conglomerates benefit from. However, a marketplace for agricultural surplus did exist in pre-information age Europe. In fact, large cities and towns (where food security was a real concern) spent lots of public money to secure and store grain, and even set up grain doles. Smaller communities instead had to make do with what resources they had, and were subject to the fluctuating prices in marketplaces, in addition to being more limited in which marketplaces they could realistically access.

I meant to open this answer with a little vignette on what the economy might look like in the 13th century. It got a little out of hand, and constitutes the bulk of the answer which you can read in the reply to this question. I chose to write about medieval Menton, on the border between France and Italy, because it is a location with an alternative to agricultural produce: seafood. Indeed, if the crop failure had occurred inland from Menton, the local vavasour or castellan might even be approached by merchants from nearby Nice looking to sell his fisherman's catch to people inland who could afford it.

The castle of Mentone (as Menton would have been called at the time) was a fief held by the Vento family of Genoa. My vignette follows an invented member of the Vento dynasty named Alberto who has been appointed castellan. While I do not know who the castellan of Mentone was, brothers Ruggero Vento, Pietro Vento, and Simone Vento were very real individuals involved at the highest level of Genoese commerce and politics. Simone's son Guglielmo would also go on to have an illustrious political career over the course 13th century.

In my vignette, the fishing harbor is damaged concurrently with the crop failure, and thus Alberto must secure foodstuffs from elsewhere. Importantly, he relies on his extensive family network: his nephew Guglielmo will be arriving to help him in the spring. Indeed a network of friends and family would be the most valuable asset that an aristocratic person could count on; Alberto might be in a pickle, but the Vento family on the whole is rich from their mercantile ventures and other holdings, thus will be able to resupply the castle of Mentone come the spring.

But the bulk of what is needed to overcome this crop failure in the immediate term will have to be bought in the marketplace. Over the course of the stormy summer in Liguria and Provence, shrewd merchants will have secured shipments of grain from Sicily, Apulia, and even as far as Crimea. The market town closest to Mentone is Nice, and that is where Alberto will have to go to access these provisions. Once the boats in the harbor have been repaird, he could also engage one of the local fishermen to sail him further afield, to Genoa, or even Pisa (the Italian city with the closest ties to grain-rich Sicily) where he might expect to find better prices and and more choice. However, the further afield Alberto goes the fewer contacts he will have, the more expensive transport becomes, and the more time he will be away from affairs in Mentone; all things that can ultimately cost him.