It seems the only way to make Gamecube ISOs yourself is to have Gamecube with a Broadband Adapter and the game hooked up to your computer. As far as I remember that's the reason why Nintendo decided to pull the broadband adapter from store shelves (at least in Europe) and avoid piracy.
Well, I've found some images online and was really surprised that my MacBook Pro (2.4GHz C2D) can run the games I've tested at 75-85% speed at 1080p. Can't wait to try it out on a more powerful system.
As for N64 emulation, I've always used Mupen64Plus on Mac and Project64 on PC. The last time I've used sixtyforce, it had the Garret watermark in the picture and you couldn't adjust screen resolution or use a controller.
Try HJsplit or 7zX to rejoin your files and rename the joined file to if it has no extension.