Another in "have learned not everyone knew this, make article about it"
YouTube videos are often good sources of training data: A few companies have put their old cartoons (and a handful of anime) on YouTube for free (or, at least, as "free" as anything on YT), some trailers/previews (especially in conjunction with promo art) can often be enough to make a LoRA before a series even airs, it's the primary source for a lot of independent animation,and there's a lot of video game cutscenes hosted there (I wouldn't recommend for pixel art stuff due to compression, but drawn cutscene stills, prerendered cutscenes, and some in-engine cutscenes are all usable as training data). Unfortunately YouTube displays its seek bar when paused, and trying to get a particular frame by printscreening while the video is playing it impractical. This is easily avoided by using an extension to get clean screen shots. I can't recommend any particular one, but any that (still) works should be able to get you good, clean, screenshots.
Bonus tip: You can use "," and "." (the keys that make "<" and ">" with shift) to frame crawl on YouTube while paused.