Where to find material to create video for YouTube?

I have a few social account with a decent following in a particular niche. Now I want to expand the network to YouTube.

I was planning to look for some creative common videos to edit together, put a nice opening, and so on, but cc videos are a limited resource.

Is it allowed to grab videos from other YouTube users, edit them (from just adding an opening, to create a video with just some parts of the original, or create a nice montage of multiple videos), and repost them crediting the original author?

No (in theory), you have to abide by fair use. Although it depends on whether youtube actually enforce it or not, since the majority of compilations are other peoples videos put together with a royalty free ukulele placed over them. Youtube have bots crawling for duplicate content, so if you rip it from other video sites which don't have it uploaded there you should get away with it. If you're going to steal videos off youtube I would recommend you never do it from someone who is partnered with an ad network, as they go out their way to take down such videos.

Google creative commons video. They are copyright free and can be used for monetisation. You can edit them as you wish too,

Yes u can do this with giving credit of original video

Why not just make your own videos

I don't think you can post someone else videos.