1.) A really cheap GBA SP? The case doesn't need to be in good shape, heck, it can have a smashed case, broken hinge, missing buttons, and dead battery, as long as it runs games and the screen isn't broken.
2.) A Gameboy classic. (The old gray kind. with the 'cart-locking' power slider.) Unlike the GBA SP, I don't care if it works at all. The circuits can be fried, the screen leaking, it can be flooded with water for all I care, as long as the case is ok. The cheaper the better, of course.
3.) A GBC Gameshark. Doesn't need to work, just needs to be intact. As before, the cheaper the better.
Yes, it's probably more than obvious what I'm planning, I don't really care, as long as I can do it somewhat cheaply. :)