http://www.raphnet-tech.com/products/atari2usb/index.php
Is this what you are looking for? This site has a lot of adapters for many controllers. Kinda pricey though and not always the cleanest looking but they get the job done.
I know those exist, but Mayflash makes a number of less expensive options that allow use of these controllers on PC or Mac via USB. I was just wondering if anyone could confirm a model that has been tested to work with nintendont.