If you've run out of tomato paste, you can substitute passata or crushed tomatoes. Your sauce may not be as thick, but it will still taste great.  You can also make your own paste from scratch. Blanch, peel, and puree 2-3 ripe tomatoes, or use 1 cup tomato puree. When cooking, make sure to simmer the puree until it reduces to ⅓ its original amount. This might take a bit longer to cook, but it's important for the right consistency.