Growing tomato sauce from seed allows the gardener the greatest possible selection of sauce tomato varieties.
Below is a small sampling of sauce tomato seed sources and their online links. These are the ones from which I order consistently because their seeds are organically-grown, open-pollinated and non GMO. I feel that it's important to support seed diversity in this way.
Heirloom Seed Company is based in Half Moon Bay, California. They carry Franchi Old World Italian seeds directly from Italy. All seeds are heirloom, open-pollinated and non GMO. Their seed packets contain the most seeds I've ever seen in a seed packet and their germination rate is estimated at over 90%. The Franchi tomato sauce varieties I grow are: Principe Borghese (classic Italian drying tomato), and San Marzano (classic Italian sauce tomato). Both do extremely well in my Pacific NW climate. I also have great sucesss growing Franchi basil, peas, green beans and Tromba D' Albenga summer squash.
Seed Savers Exchange always seems to have the most beautiful cover photo on their catalogue. All of their seeds are heirloom, open-pollinated, non GMO and each has a story. Seed Savers is my source for the Speckled Roman Tomato (super meaty, very large), Black Sea Man (Determinant, Siberian, super-sweet and tasty) and Black Krim (Indeterminant, prolific, delicious).
Restoration Seeds is based in Oregon. I grow their open-pollinated version of Sungold, a cherry tomato that is great for snacking on while working in the garden.
Territorial Seed Company is based in Oregon. They carry a variety called "Windowbox Roma" that is designed for container-growing. Sauce varieties that I have grown successfully are Saucey, Oroma, Viva Italia and Ropreco Paste.
Filaree Garlic Farm is my favorite source for seed garlic because they are based in Washington State, so I know their varieties will grow well in my garden.