Stock footage

Libraries sell it either royalty-free, where you pay once and use it within the licence terms for as long as you like, or rights-managed, where the price and the permissions are set for one specific use. Free sources exist too, though the licences vary and the clips turn up everywhere.

Two warnings worth heeding. Read the licence for editorial-only restrictions and for whether it covers monetised or sponsored videos, because plenty of free clips do not. And expect the occasional Content ID claim, since material used widely elsewhere sometimes carries claims from other uploads. Keep your licence receipts, because disputing a claim is much easier when you can produce one.