There is an original Nintendo speaker on
eBay right now, though it is more expensive than what I would pay for one. You are talking $37.50 (BIN price + shipping). You can get an extremely high quality 6.5" or 7" midrange driver for that kind of money, such as the
Peerless CSC-X for $43.75 (8 ohm home audio version) or the
4 ohm car audio version for $38.75, those same drivers have been used in home speaker cabinets costing hundreds or thousands of dollars. That driver will handle 150 watts RMS (a lot more than that if you keep the bass out of them, I have pushed 450 watts RMS to similar drivers for years).
You don't want a high end driver like that if you are not going to put a crapload of power to it though, if using the stock amp that is mounted on the Nintendo monitor, you would be better off with a stock replacement or the one that Ken Layton linked to. You would want to give that Peerless driver
at least 100 watts RMS. The cheapest way to get that is buy the 4 ohm car audio version of the CSC-X and use it with a 25Wx2@4 / 50Wx2@2 / 100Wx1@4 car audio amp bridged 100x1@4. It would sound really, really good with a setup like that though.