To search by source, first go to the Advanced Search Tab and select the Source/Publisher pulldown from the search boxes.
After you select that, the List box will appear and you can click to see the categories.
Select the category you want, and the blue box will be automatically filled with the organizations in that category.
Make your selection by clicking on it, and it will be inserted into the Search box.
After you've selected the organization, click Go.
Your search results will appear.
You can use the filters on the left to further narrow your search.