Where can I find the tool used to replace a leather button on a leather tufted sofa?

You would be looking for tools that concern upholstering furniture. The following links would be places where you might find such a tool.

Upholstery Tools, Kits, and More (http://www.perfectproductsonline.com/upholstrytools.html)

Upholstery Tools Buttons - Google Search (http://www.google.com/search?hl=en&q=Upholstery+Tools+Buttons)