My backgrounds (4 ink colors/300dpi) are from Backdrop Warehouse. I'm very satisfied with them. True, the website is extremely busy and at times difficult to navigate but there is a wealth of information there. Every order is confirmed with a telephone call. It is true it takes some adjusting to the phone manner and approach of the owner but in part it is because he believes you will have absorbed a good bit of the info on his site. Let's face it, you probably should spend a fair amount of time getting what is right for you--these are not cheap so planning ahead makes sense. I had a lot of linear feet to cover (parts of 4 walls). I found one set of agriculture scenes that comprised 5 separate photos (A-E). Maximum length is 12' and he'll print 36'' or 42'' tall for O scale.
I have permission to use the space as a train room but was not at liberty to permanently mount on the walls nor completely cover the window or remove its molding. Many recommend a frame or masonite backing on which to mount the product but I had neither the time nor the carpenter skills to do that. So, believe it or not I mounted 53' of background using poster putty and double stick 1'' square Scotch adhesive squares plus a few strategically placed staples. I was even able to get some curving of the background in the corners.
I overlapped the scenes to get a perfect match (the owner prints them that way with overlaps on the ends to be seamed) and I did use some rubber cement at the seams. I hope to write up what I did as I took photos along the way. The owner may actually give a rebate if you write an installation article for him or if you submit photos of your layout using his product.
At the moment I can't find panorama shot I took around the room but the photo I attach here gives a general idea of what I've done. Will try to post the pano shot later.