When researching political theory it is very important to choose your keywords carefully. As researchers are often looking for perspectives that reinforce and counter their views, it can be challenging to locate all relevant sources. Unfortunately, there is no "opposing viewpoint" search filter.
Reference sources, such as encyclopedias linked below are excellent sources for conceptual introductions, as well as keywords. Related terms are often listed and can help in identifying the language of the theory.
It is also common to search for works by a specific theorist, or author. UBC's Summon search is the best place for this type of research as you will find all of the print and digital copies of works available through UBC Library.