I make my own backscenes. This can be applied to all scales and is very cheap. I use GIMP free photo software to do this.
First I find the right photographs. For G they need to be quite high definition, for N that doesn't matter so much. For my indoor US based layout I went through my holiday snaps of US roadtrips and found useful photo. You can of course get photos from the internet but this is best avoided if you are going to exhibit the layout. I then shop the photo to make it what I want. Sometimes I start with a nice blue sky and then lasso various bits of other photos and plonk them on top of the sky. I also use the clone tool to smooth out any joins. Eventually I get a jpg file that is suitable for use as a backscene. Once I have this I send it to a printer to be printed. I use an online company called Solopress. They will print a 60" by 40" poster for around £20 including VAT. I find you can get quite a few backscene strips on a 60 by 40 poster. I have also used a guy called Geoff Readman to print backscenes. His prints are better quality print and can be as long as you want so no need for joins. They are a bit more expensive but no joints is good. Geoff's email is
geoff@acompleteservice.co.uk. He is a fellow modeller so understands what we are doing.