You are in luck.
I remember running across this site when I was trying to find a replacement DVD for my XBox.
Replacement Gamecube Laser thingy
You'll also need a special tool to crack the case. One site says it's just a standard torx, a full set might run a couple of bucks, another says it's sort of triangle shaped, which is going to cost like $15, I can't say for sure, because I'm at work and don't have a gamecube sitting in front of me.
No instructions come with the replacement part, so, you'll have to play it by ear. Here's something
about the disassembly to peruse at your leisure.
Hope it helps
At the most, all this is going to cost you (unless you find a cheaper laser assembly) around $65 + S&H, while a brand new Cube runs, what, $99? If it were me, I'd probably order the damn part because I hate to give up and love to tinker around with stuff. With a difference of about $30 between refurbished and brand new, I guess the question is...would it be worth it to repair?