There is no one source to go to for editorials. Newspapers and magazines are the most common venues for editorials and op-eds. With each news source having its own focus and its own slant, the editorials tend to assume some of that bias.
Here are a few national news sources that are broadly read:
Texas A&M's Writing Center offers support for both academic and practical writing. They provide guidance on writing an Op-Ed as well as strategies for approaching this kind of assignment: