Where can I find this video?

Correct, it's a celebrity fake of Natalie Portman, but it's not really a GIF. It's from the Celebrity Fuck Fest faked video series by Dr Fakenhouser.
This scene is from Volume 4 at 00:36 - 00:40
http://pornxs.com/celebrities-softcore/23458-celebrity-fuckfest-vol-4-natalie-portman-angeli.html

Correct, it's a celebrity fake of Natalie Portman, but it's not really a GIF. It's from the Celebrity Fuck Fest faked video series by Dr Fakenhouser.
This scene is from Volume 4 at 00:36 - 00:40
http://pornxs.com/celebrities-softcore/23458-celebrity-fuckfest-vol-4-natalie-portman-angeli.html

I think it's from Celebrity Fuck Fest, but I'm not sure.

a fake gif of Natalie Portman