The San Marzano tomato is a specific Italian tomato grown in the Campania region of Italy. The first seeds were brought to Italy in the 1770's and has flourished throughout the years. The San Marzano has long dark cylindrical and fleshy fruit which produces thick, sweet tomato sauces and pastes.
Unless you grown your own you next best choice is to purchase good quality canned tomatoes. Tomatoes used for canning have been vine ripened unlike the the anemic fresh/flavorless versions available for in many grocery stores. San Marzano tomatoes grown in Italy are said to be some of the best paste tomatoes grown.