Locate and evaluate sources for your issue.
Use Google Scholar, publisher databases, libraries, and Interlibrary loans to acquire hardcopies of your sources.
As a researcher, your working assumption should be that you can always get a source text. You not only live in a time where academic information is instantly available, you also are part of a research institution that gives you access to this information in a way that few have.
Google Scholar makes it easy to find an amazing amount of information. If you have the reference to an article printed in the last decade, there's a good chance the pdf is somewhere on the web. Often, authors will post the pdf on their personal websites.
If the pdf (of a journal article) is not freely available on the internet, it's usually available for a fee from the publisher's website. If you are part of a research university however, you will have access to the publisher's site through the university. If you are off-campus, log in to the universities VPN (virtual private network) and then search for the article. If the university has access to that publisher, you will be able to download the article.
If you are looking for an academic book, there is a decent chance it's available at the University Library (or for a non-academic book, at the public library). Search the library database to see if it's available
If your University Library doesn't have the book or article, they can almost certainly get it through an interlibrary loan or UBorrow service. Simply find the service on the University library website and request the source.
To build your library, start with the bibliography in your reference manager and do the following:
Using Google Scholar, Publisher databases, Libraries and Interlibrary loan, physically get as many as you can at the library, if not there, set the reference aside.
Valuate your sources (mark "to be read" or not based on following:)
Use the references from your sources to find additional sources (allowing you to find previously published articles). When you find a useful author, look for what that author has published recently or other articles that cite that author (allowing you to find articles published later). Add these to the reference manager and continue the process.