It depends on the topic. Your local library is always a great source for free research materials. You can use Google for online research and some libraries have online access to certain books (generally this applies to universities). If you are writing about a medically-related topic, medical journals and research facilities generally post research reports online--some for free and some for a fee. If you are writing about a place, then a board of tourism, chamber of commerce or town website may be of use. Niche chat rooms and forums can also serve as great resources and interview potential. If you give more information about what your topic is, it could be easier to narrow this down.
Y.L. Prinzel has give some great advice. Once you have 'infiltrated' your forums and chatrooms you could look at asking members to participate on an online survey. Depending on how complex there are lots of websites that will help you set one of these up completely free. Just Google "create free online surveys". These give you great stats and a USP for your ebook because it is your own rsearch findings that are being added to the book. This could be applied to any niche with equal success. Good Luck!
Ahhh let me see.... Gather questions and SEARCH! Really, the internet is full of information, just search for it.
... keeping in mind the title, I'm gonna go on a limb here, that even though the answers you received above are 100% on point, they're not exactly what you've been looking for. So, going even further on the limb, I'll suppose you're attempting to sell either the ebook, or something through it or an idea at least... ?! right! well, now we get to the part where you really gotta write down the proper questions to ask (yourself) in order for that to really become reality; a couple of them that come to mind : 1) who is my targetted reader (the more detail you put in a description, the better you'd get to know them, and gain familiarity in your writing style to them; and just as importantly - 2) what is their most pressing problem/frustration which I can hope to aleviate through the book (hint: selling them the solution; try focusing your research along those lines, I believe you'll get much further. Good luck!