I actually had the same question not too long ago. Got an SP basically for free, but it was all scratched up and had half peeled stickers on all over it. Worked great, but looked ugly as all heck. Luckily, I found out that you can actually buy a new shell for you SP.
eBay's gonna be the easiest place to find them. Just search for "GameBoy SP Shell" and it should bring up a beavy of listings in different colors. Or head on over the Nintendo Repair Hut (a quick google search would give you the link. That's where I got mine and am very pleased with it. It even came with the little Nintendo logo for the front and a new info sticker for the back.
Those replacement cases are actually pretty nice. Well, at least the one I got was. The quality is very close to the real thing (most people wouldn't be able to tell the difference). The only issues with them is that the colors sometimes don't perfectly match the original Nintendo line, but you'd be changing the entire shell anyhow, so that shouldn't be an issue. And once in a while you'll run into a screw hole that feels a bit small/tight when installing the new shell, but again, that's not a major problem. I haven't installed my own new shell yet, but I did one for a friend and it was reall quite easy. Did need a tri wing screwdriver, but those are easy to find online.
