Since open access research materials are available freely on the web, the most common way to find them is by using Google or other search engines. However, there are several search tools that specialize in open access resources.
Additionally, there are a few browser tools you can use to help you search for open access versions of an article:
Unpaywall finds and presents open access articles that have been legally archived on other websites and repositories to users who may have found a paywalled version.
The Open Access Button is a free tool that allows you to find free, legal articles with just a click and that will email an author to get a copy if one isn't available. It's available as a Chrome and Firefox browser plugin, as well as a website.