Encyclopedia Britannica Kids
Encyclopedia Britannica Kids is a great source for information in a variety of forms. You'll have to scroll down to see the search area (and you may have to deal with a few annoying pop-ups encouraging a subscription purchase), but it's worth it for the wealth of information here!
The History Channel
The History Channel has resources and allows for easy biography searching. I really like the way they present timelines in most of their biographies.
PBS LearningMedia for Kids
PBS LearningMedia for Kids has several types of resources and information on a variety of topics, including several of your explorers!
Epic
Epic has a variety of books on different topics. Some explorers are here. Others are not. Check out the collection I built for you!
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Sweet Search
Sweet Search helps students find outstanding information, faster. It helps students find good information quickly without spam.
This isn't a perfect source, but it helps!
Google Scholar
Google Scholar is a type of database. It is a little less user friendly than some of the recommended databases, but there is a lot of knowledge here if you're able to work through complicated texts.