Most N64 gamesharks that are still floating around out there do not have a functional PC port and therefore cannot be interfaced with your computer. However, I've discovered a relatively complex method of uploading large gameshark code lists to gamesharks without working ports. In order to do this yourself, you would need multiple pieces of equipment and a computer with Windows 98.
Therefore your only option is to enter these codes by hand. The problem is that entering 2000+ codes by hand is tedious, takes many many hours, and is just asking for mistakes.
So once I've finished the last of my multiplayer hacks, I'll be selling FULLY LOADED gamesharks with all of my multiplayer levels already loaded onto them. I'll likely sell them on ebay and post links to the auctions here. Check back soon for updates.
If you just can't wait, contact me (visit the LINKS page) and I'll see what I can do for you.