I don't think there is a single website or book I can point you to, but since I've spent a lot of time on similar searches, I'll try to give some general pointers.
First of all, keep in mind that the franchises you mention in turn got their information from somewhere else, so you can start by looking at their influences. Let's take Dwarves in Middle Earth: Tolkien got their names from the Poetic Edda to start, and used that plus other germanic mythology to create their characteristics, before putting them through the filter of his own imagination and free associations. Look for books or interviews with creators to see what the sources of their fantasy races were, and keeping moving back until you get to one or more folklore sources.
Second, keep in mind that folklore is messy, there is rarely one codified source that says what a dwarf is like. If there is such a source (like the Poetic Edda), keep in mind that this source was compiled by someone, taking centuries of sometimes conflicting versions of stories and trying to weave them into a coherent narrative where there was none originally. Therefore, know that you will never find a single definitive origin of a folklore creature. Instead, look for how the description of a folklore creature varied over time and place, and choose the version or versions that appeal to you most as a writer.
The first place I often start in trying to find out about how so and so author came up with a creature is just googling something like Tolkien dwarves origin and seeing what comes up. If you can at least pinpoint a cultural folklore that the creature came from, you can search your library or worldcat.org for books about that culture's folklore. Start with a survey work, and from there you can follow footnotes to find specific stories about the creatures you're interested in (hopefully translated!)
