Firstly download Gamepad Companion here.
http://www.carvware.com/gamepadcompanion.html
That'll allow you to use many different controllers with your mac.
And here is a list of gamepads that work with this software:
Gravis Destroyer Tiltpad
Gravis Eliminator AfterShock Runmble GamePad
Gravis Eliminator GamePad Pro
Gravis GamePad Pro USB
MacAlly iShock
MacAlly iShock II
Microsoft SideWinder Game Pad USB
Microsoft SideWinder plug & Play Game Pad
Microsoft Stratogoc Commander
nostrom n50SpeedPad
Thrustmaster Firestorm Digital 2
Saitek X45(should work with OS 10.2 but has not been tested yet)
Saitek P750(Requires OS 10.2 and GamePad Companion 2.1 or later)
And for my liking I'd stick to SNES 9X for the SNES and Rock Nes for the NES. Though you might like Nestopia too. And most of the above gamepads will work with those emulators. You just have to go to the shops and pick the one from that list you like the most. I think the are all USB or wireless. So they should work just fine.
Finally, downloading the app here might help you out too.
http://www.bannister.org/software/ee.htm