I wish you luck. Just look for a system with the moulded, not printed, "eject" button and two rubber feet on the bottom. They don't yellow, although sometimes the area around the controller ports still can.
I managed to find a perfect condition original SNES (the first production run with the separate sound module) at a swap meet. It was loose with a bunch of controllers and cables, for sale by some random old man, so I'm not sure how it managed to make it 20 years without being damaged or yellowed.