Databases are subscription resources that bring together articles and other types of research materials into one place with a sophisticated search engine.
There are many, many databases you can use that provide access to journal articles, book chapters, book reviews, conference proceedings, dissertations, and more related to your environmental policy topics.
Databases from the same vendor/publisher may be searched together to weed out duplicate materials. See the page: Searching Multiple Databases
For example, Agricultural & Environmental Science Database, PAIS International and Archive, and Worldwide Political Sciences Abstracts may all be searched together (ProQuest databases).
Academic Search Premier, GreenFILE, International Political Science Abstracts, Business Source Complete and EconLit may all be searched together (EbscoHost databases).