It's obviously not a source, but I keep thinking of the forest in Robert Holdstock's Mythago Wood, where the forest is the actual portal through which myth is embodied. I think they're both working from the same source, though. England (does? did during Tolkien's life, anyway) still have forests that were older than human memory, after all, and fairy tales are always full of forests. (I, who grew up in the American Midwest, always longed for forests like that.)