Through EZ Prints, you can purchase 3inch or 4inch buttons. Beware the trimming that occurs, however, because you don't want to lose an important part of the photo in the printing process!
This product page has info on the product itself, but here are some tips to help you anticipate the trimming that occurs:
In the shopping cart, we'll show the "trim area" in blue. We allow a quarter inch of cushion on each edge to print the 3 or 4 inch buttons. What we don't show (sorry!) is a preview of the round shape of the final product. I've added that to this image, as a yellow overlay:

This image is well suited to be made into a button, but many images are too closely cropped or contain text much too close to the edge. So the final 3 inch button from this image will look much like this:

If you are a photographer selling buttons, we highly recommend using the proof-delay feature. It allows you to adjust cropping and completely replace an image as needed before orders begin processing.