I'm not sure what you mean by "model", but usually buttons are created as part of a UI library. As part of the process, designers choose a particular visual style for clickable components such as buttons. This includes button shape (rectangles, rounded rectangles, pill-shaped, etc.), border style, whether to make it 3D with a bevel/drop shadow/gloss/etc., whether to give it texture, what font to use, how the different button states (normal/hover/depressed/active/disabled) look, as well as the color scheme of the UI.
There are pre-made UI libraries/themes out there, and sometimes designers will modify button templates, but with something as simple as a button, you typically just create them from scratch. And even if it's a more elaborate UI theme, you still need the buttons to match the rest of the UI components, so you'd probably create them from scratch anyways.
You may be thinking of pre-made buttons that you can download the images/CSS/JS for. Some people who aren't graphic designers or aren't good with CSS find them useful. You can find many of these by googling "CSS buttons" or "CSS3 buttons". Some of them can be easily customized for different colors/backgrounds/shapes.