If you're using it for sporadic testing, you might want a wired controller so that you don't have to worry about batteries or turning the controller back on if you haven't tested a build in a while. If that's the case, then I'd highly recommend the iBuffalo SNES style controller.
If you absolutely need an NES controller (and have some basic soldering skills) RetroUSB sells USB converter kits that can turn OEM NES controllers into USB controllers. You also might try using a NES to USB adapter, if you don't want to bother with the soldering.