I pretty much did the same thing as brodyboy did; go to Google Images and you can more than you'll ever need there.

Remember that the cool thing is that NevoStudio auto-resizes backgrounds, so it doesn't matter what the size of the picture is you download.

Buttons are another story though, their size appears in NevoStudio the actual resolution that it is. Also remember that in order to create buttons, you can either use one or two images. If you use one image, the button will not have that cool "pressed" look when you go to press it, but if you use two images (and obviously the images have to be a little different), you can create that look. If you go with two images, both images have to have the same exact file name except that the "upstate" ends in _0 and the "downstate" ends in _1.

Just go to Google Images, you can't go wrong.