You buyed a new 4K HDR tv and wanna enjoy the Ultra HD?
Over the streaming offer of various services (Netflix, Amazon Prime Instant Video) and 4K Blu-Rays you can also watch 4K contents for free.
On Youtube and Vimeo you can find many videos (timelapse etc…) in 2160p quality within a simple search.
The following sites contains demo/trailers videos to download:
If you have satellite you can also watch 4K television, Astra started broadcasting UHD.