A working paper, or research paper or pre-print, is a work in progress that may or may not be forthcoming in a law review. The Social Science Research Network (SSRN) and The Berkeley Electronic Press (bepress) are two databases that host hundreds of thousands of these working papers. Working papers uploaded to these sites are in theory their first appearance on the scene of legal scholarship, before being published or uploaded elsewhere.
The bepress Legal Repository is a website run by The Berkeley Electronic Press that, among other things, offers working papers from scholars and professionals on legal topics. Another iteration is the Law Commons, through the Digital Commons Network.
(SocArXiv may be an alternative.)
These are free resources that you can use to narrow your search before you go to licensed databases such Lexis or Westlaw or indexes. You can also check bepress and SSRN to make sure your survey of law review articles is as current as possible.
You can find content broken down by subject by visiting the Browse Subjects tab. The legal repository articles are indexed within 101 legal categories.
With the bepress search box, you can type in search terms and the system will search the titles, abstracts, and full text of articles in the repository. With the advanced search function, you can narrow your focus by searching within particular categories, such as author name, subject area, institution, date, discipline, and full text.
Search: Within its Advanced Search page, you can narrow the "networks" to law. Full text search is in beta mode and currently doesn't work very well; instead, search via a combination of author, title, abstract, keywords, and date. A better approach may be to use a Google's advanced search feature to search its entirety - site:papers.ssrn.com. You can also set up a Google alert to track your topic going forward. (Ex: "legal research" site:papers.ssrn.com)
Browse: You can also browse SSRN, but you have to drill down to narrow it to the legal content within LSN. You can click the + sign next to "Legal Scholarship Network" to see the different subsets. Many of these subsets are groupings of working papers from different institutions. However, this browsing portion also displays "LSN Subject Matter eJournals." Although called eJournals, each one is effectively a more narrow legal subject database. Clicking any one of these will bring up a "Search Within Results" search box. "Search Within Results" will search title, author, abstract body, and keywords fields, but not full text apparently. If the paper is free, it will have a blue dog-eared page icon next to the title. If you see a red dog-eared page with a dollar sign ($) next to a title, it is available for a fee.