I understand you're looking for a complete NES ROM set, but I have to clarify a few things:
ROMs and Copyright: Many ROMs are copyrighted and downloading them without owning the original game or having permission from the copyright holder can be considered piracy. However, there are some games and demos that are publicly available for free.
Legality and Ethics: Before downloading any ROMs, ensure you're aware of the legal and ethical implications. Some sources provide ROMs for games that are no longer commercially available, and in such cases, downloading them might be seen as a grey area, especially if you own the original game.
Safety and Security: When downloading ROMs from third-party websites, there's a risk of downloading malware or viruses alongside the ROMs. Always ensure you're downloading from reputable sources and use reliable antivirus software.
That said, if you're looking for a complete NES ROM set for archival or historical gaming purposes, and you're aware of the legal and ethical considerations, here are some general tips on how to proceed:
If you worry about the morality of downloading 1,200 games for free, there is a path to a "clean conscience" complete set: The Dumper's Path.
You can buy a device like the INLretro dumper, Analogue NT Mini Noire, or a Retrode with NES adapter. These devices allow you to physically insert your old cartridges and "dump" the ROM to a USB drive.
If you go to garage sales and buy 50 sports games for $1 each, dump them, and then resell them—you now legally own that digital copy. Doing this for 20 years yields a full legal set. Most people don’t have that time, which is why direct downloads are the norm.
Because NES ROMs are small files (usually 128KB to 512KB), hackers love to hide malware in "ROM pack" executables. Never download a file that is a .exe, .scr, or .bat. A real NES ROM ends in .nes, .fds (Famicom Disk System), or .zip. Many shady websites will tell you "You need to download our ROM Loader" – that is a virus.
This is where the conversation gets uncomfortable. Under the Digital Millennium Copyright Act (DMCA) and the Copyright Act of 1976, downloading ROMs (read-only memory) of copyrighted games is illegal. Period. Nintendo is notoriously aggressive in protecting its intellectual property.
You have likely heard the rumor: "It is legal to download a ROM if you own the original cartridge and delete it after 24 hours." This is completely false. There is no such law or precedent anywhere in the world. It is a fabrication from early internet forums to justify piracy.