Choosing nature b-roll
Match the energy of the edit: slow, wide shots (a calm sea, drifting clouds) sit under voice-over without competing; close, fast shots (breaking waves, a timelapse storm) work as transitions and openers. Keep the light consistent across a sequence: golden-hour shots next to midday ones look stitched together. Shoot or pick wider than you need, since a 4K source lets you reframe and punch in on a 1080p timeline.
Where free nature footage comes from
Wikimedia Commons holds hundreds of thousands of freely licensed videos, many of them nature and timelapse clips, each with its licence recorded per file. NASA's libraries add Earth seen from orbit and its weather. The Internet Archive's public-domain films include mid-century nature and travel reels. Free stock sites such as Pexels and Pixabay have large modern nature libraries too; they can't be searched automatically, so each search links straight to the same query there.
Licences for nature footage
Landscapes rarely need releases, which makes nature one of the safest kinds of stock to use commercially. Check the licence itself: CC0 and public-domain clips need nothing, CC BY needs a credit line, and CC BY-SA would require your whole video to be released under the same licence. Recognisable private property and people in frame can still need releases for ads.