Screen protectors come in many sizes for cell phones, electronic games, iPods, etc. Some are pre-cut to fit specific models and some are just a general size for trimming.
I have some that are a bit larger than the screen on my DB which are easy to trim. Telephone shops in Bangkok at MBK (fourth floor) sell every conceivable size, but I would guess that most are available elsewhere as well. My daughter bought some for her phone and Nintendo DS via Amazon (in the U.S.).