Classic NES

You could get a emulation device specifically for older games, or do what I did. Picked up a playstation 3 for like 100 bucks, you can soft mod it meaning all you need is like a flashdrive. You can just download a nes emulator onto the ps3 and dump your roms into it. Moding ps3 is great because you can just dump the whole ps store onto it and all of your old roms.

as other have said, if its for replay and walk down nostalgia lane an emulator device is the way to go.

if you want to get them an actually real deal thing as more of a cool thing to have then probably your best bet is still things like ebay. other good option is find your local card game/comic book store. a lot of them dabble in old game and game consoles and they could have one.

just remember most modern displays wont have the right hook ups for an nes and even after adapting its going to look like poop more than likely.

Allow me to present a different option:

Get an actual NES and an Everdrive. That way you can play on original hardware but have all your games on an SD card that goes in a cartridge.