Most of my 40k posters are event posters from my local store from years ago. GW used to (still does?) provide stores with posters advertising in-store events, new releases, etc.... and once the event is over you can sometimes get them from the manager. Or sometimes they get given away as prizes.
As for official posters, Warhammer Art
sells some. Not sure if they're what you're looking for, but they might have something suitable.
I don't know of any third-party vendors with GW imagery. I suspect they'd be fairly rare and/or short-lived, given GW's stance on protecting their IP.