Your first source of data is existing academic and professional research. For example, one source might be: http://dl.acm.org. To access research databases, you may normally need a membership in an educational or professional organization. However, many libraries fund free online access to the databases. For example, my local library's page for that is http://www.spl.org/library-collection/articles-and-research/science-and-technology
Some research is shared publicly by the author in all or in part, or open-sourced. You can search research like that via https://scholar.google.com/
You can also conduct your own contextual research around the topic. For example, if you are working on a project for a plumbing business, you could find study participants who are plumbers, or people who recently needed to find a plumber. From there, you can find out their current needs, areas of frustration, tools they are using/not using and why, etc. It's actually completely ideal to start research before there is any design to show people... it helps you get a better sense of what you could create to help solve people's problems and/or encourage whichever behavior you want.
Additionally, you can research competitors... run studies using their websites to help you understand what's working well, what's not working, common design trends or other trends, etc... as well as reading existing research about competitors.