What you get: An assortment of blank buttons (normal, press, disabled), in a variety of sizes from 23×23, to 600×23, in BMP, GIF, JPG, and PNG formats.
A demo of a Windows application with a custom form using these buttons can be downloaded here. No, sorry, I am not making the source code available, but if you think about it for a bit I am sure you can figure out how to do it in whatever language you are using.
Released under a Creative Commons Attribution license (CC-BY), it is suitable for personal, non-commercial, and commercial use. All you have to do is link back here to give credit.