If you are writing for an on-going series, then the main characters and setting are already set. What's left that doing research could help with? Story ideas and a deeper understanding of the story.
If the show mainly deals with social issues, then you'll want to 1) think of a fairly original one, and 2) use quality sources for the deeper understanding.
If you aren't already given the issue, or interested in getting a broader view of some possibilities beyond ones you already know, here are a few places to peruse.
There's no shortage of information. The problem is filtering for the quality information worth your time. Ask yourself:
The Library pays to access information that has gone through some filtering, either newspaper or magazine articles that go through an editorial process before being published, or scholarly sources that have gone through an academic peer-review process. But with google searches, you have to do identification and filtering.
Occasionally you may need to consult scholarly sources, but most of the time you can gain a deep understanding of social issues through magazines, newspapers and online media that aims for quality.
Major Newspapers
We have full text versions (via ProQuest) of the New York Times, Wall Street Journal, Los Angeles Times, and Washington Post. (The New York Times and Washington Post offer discount subscriptions for students, $1/week for online access, not free as I mentioned in class. Washington Post changed their minds on that, apparently.)
Major News/Opinion Magazines
Rather than searching through thousands of magazines, this link limits to some of the ones more likely to have lengthier articles on social issues.
When you're adapting from a source, such as The Handmaid's Tale, you'll want to know the work really well, and that includes knowing about the author.
Look for interviews with Margaret Atwood. You can use ProQuest Central to find ones in magazines, newspapers and maybe a literary journal or two. (Try "margaret atwood" and "interview* with" -- I got 130 results. 20 if I included the term handmaid's) Or Google Videos to find archived videos of her at book readings, press junkets, etc.
Other Dystopian Stories
Here's an example of an overview of dystopian literature from The New Dictionary of the History of Ideas.
If you want to search through articles, I would suggest also using Gale Literary Sources (see above) for dystopian literature. Since you are keyword searching the entire text, you can be very specific and look for analysis of dystopian and race, for instance. Or whatever plot you were thinking of doing.
Biblical References
[forthcoming...]
Since Preacher is an adaption of a comic book, you want to know your source material well. (Wikipedia says 66 issues, 5 specials and 4-issue miniseries! How many of those plots did they get through in Season 1?) Preacher and the comic book creators aren't high-brow enough to be in Gale Literary Sources, so I would stick to looking for interviews of the creators for any insight into their thought process, as well as any other comic series they have done.
Also, look for other comic book series that are similar in tone to Preacher. Try Wikipedia, but also try this print set: