Back in the early 2000s I got nostalgic for my old NES (Sold it in a garage sale stupidly when I was young). Started going around to garage sales, flea markets, and occasionally Ebay. I started off just hunting for games I used to have and once rented, then it turned into a full on collection. I'm sitting on 600+ unique carts now with another 150ish to go to complete the collection. I kind of quit collecting once kids came around, just didn't have the budget for it now...and unfortunately a lot of NES stuff has really increased in value in the last 10-15 years.
If you're looking for a deal, flea markets and garage sales are going to be your best bet, although these days you're probably unlikely to find NES games at a garage sale unless you see them listed in a newspaper ad or something. Flea markets still sell NES games, just keep in mind that as online auctions got more and more popular 15-20 years ago people started getting more and more knowledgeable on what certain games were really worth on the market. It was always fun when you found a truly rare game sitting in someone's box of junk with a $5 price tag, harder to get those gems now even at a less than ridiculous price.
Super Dodge Ball and River City Ransom are definitely great two player games. Mario 3, Contra, TMNT II, Super Tecmo Bowl, Tengen Tetris (Harder to find, better than Nintendo's version), Dr. Mario, Rampage, Base Wars, and Battletoads (Very tough though) are all other options. Most of those will be easier to find too, aside from Tengen Tetris, but you can always get the original too.
It's a one player game, but Fire and Ice is a fun little game that everyone can enjoy. It's a puzzle game that my wife really enjoyed back in the day, and I'd help her when she got stuck. Unfortunately it looks like it goes for stupid money on Ebay now, you used to be able to get it for about $20 in the mid 2000s. Maybe just get a ROM of that one unless you get lucky and find it cheap.