There is no board devoted solely to pizza sauce recipes. In many cases, the pizza sauce recipes are co-located with particular pizza styles on one of the pizza making boards or, if the sauce recipes are general in nature, they might be found in the General Pizza Making board.
I suggest that you use one of the forum's search features. For example, you can use the Advanced search feature at http://www.pizzamaking.com/forum/index.php?action=search;advanced
. With that search feature, you can limit the boards on which you would like to conduct your search, or you can search all boards (but at the risk of getting too many hits). As an example, if you are looking for a NY style sauce recipe, you can limit your search to the New York Style board.
The second search you can conduct is to use the Google Custom Search feature that is accessible at the bottom of the pizza index page. That search can be conducted just as any other Google search but the search will be limited to only the forum content. Your search string can be as long or as short as you would like it to be.
Some people conduct both types of searches.