You can easily find original NES systems being sold at places like Amazon, eBay.com, and many other, smaller websites. You can also find many good package deals too, like where the console comes with all 3 of the original Super Mario Bros. games, the zapper gun, 2 original controllers, etc. But those package deals will obviously cost more money though. Also, you will find that many of the NES systems being sold on eBay and Amazon have been refurbished, which is great, because it means that you will not have to fight to get the system to work every time you insert a game to play.
Most of the refurbished NES systems have brand new 72-pin connectors, which was definitely the weak link with the original NES systems. The old worn out 72 pin connectors would not grip the NES game cartridges very tight, and then the game would have to be taken out and re-inserted many times before it would work. And who wants to deal with that?