I typically get panoramic photos that are copyright free and royalty free off the internet and then have them printed in large format. There are many places that do this online and I believe Target and Walmart do it too. It's fairly inexpensive.
I try to find shots that go well with my scene and can transition with little effort or can be hidden easy. I also cut away any sky and clouds. I painted my wall sky blue and painted my own clouds with spray paint and templates that I cut with scissors. There are threads on OGR on how to do that.
This is actually two photos with a tree strategically placed to hide the seam. The lichen bushes also hide where the layout touches the wall.
This one was simple. The photo is an Iowa cornfield. But I'm trying to pull it off as Lancaster County in PA.
There should be a tractor here harvesting this crop. I had to take this scene apart and am still putting back together.
This is actually a photo over a photo and will be the next scene I work on... a campground. When I cut out the sky, I use a black sharpie to hide the white edge of the paper. It also helps provide some separation and the illusion of depth.
These next shots are the same scenes, but show the seam hiding techniques or view breaks.