You can order directly from Escalaon, or look for a brand of ground tomatoes in your grocery store. If you can't find ground, then used crushed (S&W, Contadina and Progresso are good). Even 6-in-1's can be bitter, and other brands can be very bitter, so I always add a tsp of sugar to a 28-oz. can.
Personally, I like an uncooked sauce better than a cooked one--you can experiment with both to see what appeals to you!