I have my students create research notecards, where they write the quote or statistic in a specific format, need a number of different sources and types of sources, then I go through the notecards and reject the ones that don't meet the criteria.
Then, instead of typing the paper (read: cutting and pasting from some other website), I have them write it in class, in front of me, in pen, during a class period. Then they have to express everything in their own words, with their supporting info. The fact that you look over the notecards ahead of time is the key to preventing plagiarism because you can limit what they put on them.
I never give worksheets as homework - ever. Stduents will cheat off of each other every single time, even the best students. They're almost worthless as learning instruments.