If you have a variety of original hardware that you're looking to have plugged in at all times, BlissBox is the best option out there hands down. If you only have a couple of controllers, or you don't intend to have everything plugged in at all times, there are also a variety of single adapters out there (I've had good luck with Mayflash in the past, but I know others who have had issues, YMMV).
FWIW I'm not a fanatic about using original hardware for emulated systems, but there are certainly some systems out there for which original hardware is really the only way to make it seem "natural" (N64 is one huge example of this).