The most common name in Argentina is pimienta de Jamaica. In other Spanish speaking countries its called Pimenta dioica, pimienta gorda, pimienta guayabita, pimienta dulce, pimienta inglesa or pimienta malagueta.
I have found pimienta de Jamaica in several dieteticas and in Mercado Central in Mendoza.
A tip: When you want to find the name of e.g. a spice (and lots of other names), find it on en.wikipedia.org (or your native language wikipedia.org), then select Español to see the Spanish name for it (e.g. http://en.wikipedia.org/wiki/Allspice -> Español which sends you to http://es.wikipedia.org/wiki/Pimenta_dioica).