You can purchase licences to use cool backgrounds from various websites. I use istockphoto.com, dreamstime.com and bigstockphoto.com. Once my customers pick out their color theme, I just go look for backgrounds and then get the customer to approve which one. Then I can do my graphics from there, keeping the color tones the way I like.
These type of sites you buy credits and the graphics will all have different prices for different sizes. Some of the sites have a time limit too, so if you buy extra credits make sure you use them before they expire.
When you search for backgrounds try using words like seamless or tile. You want to make sure that you can load a smaller image format and that it will be seamless when it is applied to the background.