1. The library - I know, you've been told this by ten thousand people already, but it will help. You HAVE to use it right though. Walking in, and scanning the squat with no clue what you are looking for will serve NO purpose. The library will be organised into subjects of interests, so use this to your advantage and go in knowing what you want. Find the subject and the specific interest before you go diving into books head first. Or better yet, ask the Librarian where your field of interest can be found. Once you have found a selection, look at the contents and index to narrow your search, and see if the books are actually relevant or if the titles are WILDLY misleading. For example I was reading what I thought was the 'History of London' when in actuality it was based on migrating pigeon population. Ok, so now you have your books (possibly about 2-5 depending on how broad your search is), DO NOT read each and every page of these books, you will either die of boredom or old age. Scan the contents again and find chapters that are appropriate, or even better, if they have a references page, find the relevant ones and you have your sources, BOOM. (Obviously check they are legit and make sure they reference page numbers). Failing that, look at the index/ glossary and that should give you somewhere to start.
2. Talk to friends - First I would check they are reliable and/or have a similar aim/topic to you. They can recommend books, or if you are doing the same thing, share sources. Of course, you will still have to find some of your own sources otherwise your work will be dismissed for plagiarism.
3. Teachers/Lectures - Chances are they will know a lot more than you... That's why they are stood at the front. However you can't just directly ask for help, because they WILL say no. But instead ask for reading recommendations for your topic "just to get you started" or "to add a different perspective." Big words are bound to help you here.
4. Famous people's speeches - By famous, i don't mean Beyonće... unless that's what you're talking about? Anyway, what i mean is prominent historical figures, world leaders or pioneers in the area of work you are researching. You can use the internet to find these, just type in the name and what you're researching, even wikipedia is good because it has references at the bottom of the page that you can use to find where facts and sources actually come from.
5. Use others work - Like siblings, people who have done similar, pieces of work that the school/uni have kept as examples or find mark schemes for your work and the exemplar sources. Don't just copy it though, because again, plagiarism with screw you over. Use some of the sources that you can't find yourself and maybe get an idea of some of the books they've used. Just make sure you explain everything you've taken, by yourself.
6. University research websites - Regardless of whether you are in uni or not, the research they do there is great for finding sources. If you need detailed facts, historical reports and journals this is definitely where you should look. Just type in what you're looking for and uni websites are usually the first ones that come up.
7. Just Google quotes - Seriously if you've found a quote but have no idea where its from, just Google it and something will come up.
8. Google books - An extremely helpful Google resource which is EXCELLENT. Type in anything and a book will come up. If you find a book and want something specific, type it into the search box and it will highlight your search whenever it appears in the book. Sometimes school textbooks will come up but don't discount these because they tend to cite their sources quite well.
9. Google Scholar - I'm not such a big fan of this one but it's great if you're not just looking for books. Court cases, laws, journals and stuff like that is where scholar really excels. It also have a citation importing feature... I don't know what that is but it's one of their main advertising points and it sounds kinda cool.
10. TV Documentaries - There is always something useful on TV. Lots of journalists, researchers and reporters make documentaries which are especially informative and beneficial to watch, because they are PACKED with great information.