If possible, try to peek into the kitchen and look for sauce cans. Most often, Pizzerias are using a base for their sauce. Examples: Stanislaus Foods, who makes Full Red, 711, Pizzaiolo, etc... and other brands like Escalon, who also makes many pizza sauces, as well. Then you have the many distributors that makes sauce, such as FSA (Food Services of America), SYSCO, US FOODS (now owned by SYSCO) and many other smaller distributors. You might want to look over Stanislaus and Escalon's websites to familiarize yourself with their products and labels.
Also, talk with some of the workers during slow hours of the business. Most often, people will share info with you as you develop relationship with them, including being a good tipper. If all else fails, ask if you could buy some sauce from them.
Final thoughts: I would also find out the brand of cheese that they use. You can duplicate the sauce and still not have the same taste. The cheese really does make a big difference. Hope this helps. I am sure others on the forum have ideas too.