You'd have to buy a whole new piece, probably better to buy a new GCC or take the stick from a less used controller. Or you could try something different and use the nunchuck's stick or something. You'd most likely only find the stand alone piece (might come with a few other pieces like an extra Z button ect). on Ebay, or Amazon, or whatever.
First priority should be a Tri Winged screwdriver if you don't already have one. ****'s like 3 bucks anyways.
Rubber came off my control stick from Melee days, just got used to it. Can't play any different now lmao.