Here's one for you, but be forewarned, they ain't cheap.
http://www.backdropjunction.com/
The really cheap alternative is to get some hardbaord and a gallon of sky blue paint. If you want to get a little more ambitious, you can airbrush in a couple of clouds. (We are about to try this ourselves.)
Richard
Backdrops can make or break the appearance of a layout. Photo-realistic backgrounds have the tendency to dominate a scene, causing the eye to focus on the backgrund instead the foreground. Painting your own background requires a lot of artistry, which not everyone has. Even a simple backdrop with some clouds is a lot more difficult to create than one would think. Before you invest a lot of time and money into something you may not like later on, start with a simple backdrop - just a very light blue or very light gray. You can add clouds later, if you like.
One of the very few really fantastic examples of a backdrop is on Troels Kirk´s On30 layout - have a look:
See how many different shades of blue the sky has and how many different colors the clouds have! Btw, Troels is a professional Swedish artist.
I use the ones from Walthers http://www.walthers.com/exec/search?manu=949&split=30
and Scenic Express http://www.sceneryexpress.com/products.asp?dept=1100
I usually cut the sky away from them and then glue them on the wall with 3M 77 spray adhesive. The wall is painted a sky blue before mounting them of course. You could use masonite as a mounting backdrop, paint it blue, and then glue on the scenes that you want. I usually put background buildings in front of the seams to hide them.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Make your own photo backdrops. Find locations where you can take panorama photos that fit your needs.
Many inexpensive digital cameras have a panorama setting. I have a Sony Cybershot that cost less than $100.00.
A digital camera that doesn't make panoramas can also be used. Multiple shots can be stitched together to make a panorams using a computer. There are free programs available to do this. (one iof them is Microsoft ICE). Be sure there is about 1/8 frame overlap between the shots. Works best if the camera is mounted on a tripod, but hand held will work.
A friend made a very nice backdrop on his HO layout from stitched mutiple shots. He printed it out on mutiple sheets (13"x19")uaing a large format printer . From normal viewing distance the joins between the sheets are not visable. He wrote an article for the NMRA-PCR Sierra Division newsletter:
http://www.pcrnmra.org/sierra/Feb-2014-digital.pdf
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
Here are some relatively recent related threads that may help round out the info:
http://cs.trains.com/mrr/f/11/t/221045.aspx
http://cs.trains.com/mrr/f/11/t/217998.aspx
I used a masonite backboard, painted with varying blue shades top to bottom, and Scenic Express city and industry background scenes cut out (to use the consistent painted sky on the masonite) plus foreground "background" buildings of two types: Cornerstones roughly 2" deep items plus some "flats" (photos) from Kingmill Enterprises mounted on foamboard.
Paul
Modeling HO with a transition era UP bent
backdropwarehouse.com
Joe C
A Link to Joe C's suggestion:
http://www.backdropwarehouse.com/indexbdwh.htm
Frank