Can't think of sources but have heard that you can create your own
in you can use Adobe Photoshop and a computer. I think Illustrator
or any program like it could be used to accomplish it also. You make
a graduated background using the color you want and a suitable grad
scale (long or short tonal rang, left, right, top, bottom, circular,
etc) and then print it out in the size you need.
Hope this helps.