@dalias Most of the random cheap IP security cameras on Amazon have H264 RTSP streams available.
They do usually have horrible firmware full of cloud BS and require some sketchy windows binary for initial provisioning, but you can set it up from a VM then note the URL of the stream, wipe the VM, and just access the raw stream with zoneminder or VLC or whatever.
Put it on a subnet with no outbound allowed and you're good to go.