We only use San Marzano Tomatos.
...From a small town outside Naples, Italy, come the best tomatoes on earth. Nestled at the base of Mount Vesuvius, the sun-drenched volcanic soil of San Marzano provides the perfect environment for the production of the most sought-after tomatoes in the world. Ask any real Italian restaurant owner....

....The San Marzano Difference
(Science Behind the Tomato)
Round tomatoes have between five and seven locules or seed pockets; plum tomatoes like the san marzano have two. How did that happen?
"The plum shape might have arisen as a mutation," says tomato geneticist Charles Rick of UC Berkeley. "Only one or two genes
differentiate it from the round tomato. It apparently wasn't in any of the original varieties brought into the Mediterranean, but the fact that it is pretty widely spread through the Mayan area [of Mexico and Central America] suggests that it is a very old type of variant.".....

http://www.sanmarzanoimports.com/

Here we buy them:

http://www.pennmac.com/page/24