I am making a dip that requires 'pizza sauce'. The store doesn't sell anything called 'pizza sauce' that I could find. Instead, I got plain tomato sauce. What do I need to add to it to make it 'pizza sauce'?
Pizza sauce is, presumably, whatever tomato-based sauce you'd put on a pizza. Those tend to be essentially like smooth (not chunky), lightly seasoned spaghetti sauce. If by "plain tomato sauce" you mean a jar of tomato sauce for spaghetti, you're probably in good shape. If you mean a can of pure tomato sauce, with no other ingredients, you'll probably want to add some seasoning. Basil and oregano are good bets for herbs; garlic and onion are also common, and perhaps crushed red pepper if you want a little kick.
If you're looking for a real recipe, this isn't a recipe request site, but search engines will serve you well.
In my family pizza sauce it just fresh garden tomatoes simmered for a few hours or more with a good drizzle of nice olive oil
I leave the pot lid on until the tomatoes have fully rendered down and are just lightly bubbling
Add fresh chopped herbs when serving or adding to a dish i.e. smearing on pizza base
Pizza sauce is typically much thicker than the tomato sauce you would put on pasta and more heavily seasoned. The idea is that you are spreading a thin layer of the sauce over a large area, so a thinner sauce won't work and/or might lose too much liquid as you bake your pizza. Don Pepino pizza sauce is great, assuming you can get it where you live. If you have a jar of commercial pasta sauce around, I would add basil, oregano, and garlic to it. Then, add a good bit of tomato paste to make it thicker.
Most grocery stores carry Pizza Sauce. I know for certain I have purchased a Ragù variant at Publix, Walmart, Sweetbay and Meijer on separate occasions. It typically comes in a much smaller glass jar than that of spaghetti sauce. I've seen it shelved either with the pasta sauces, canned tomatoes or in a bakery section next to pizza doughs.
Or you can just make your own, as I tend to do most of the time these days.