The best places to buy your favorite N64 games is Amazon and eBay. Several different sellers offer these titles at competitive prices, so you should be able to find exactly what you’re looking for without breaking the bank.
If you want to avoid paying for the shipping on a used game, try shopping at GameStop or another retail store that sells used games (like Best Buy). You can also check out local thrift stores; some of them have great deals on video games from time to time.