It looks like the PSP screen protectors should be big enough. Just needs a little modding.

Edit:

Apparently Hori makes the best screen covers, pus, at $5, it's half what Target charges. EBgames sells them.

All I had to do what cut a bit off the width (x axis). I didn't cut enough off but the small amount extra fit easily under the screen. There were a couple of air bubbles but I managed drag them out with my stylus.

The height is actually a fraction too big, which is why I didn't want to try cutting it. Perhaps as a result of this, there is a small hairline bubble along the top of the screen. This may have been my fault as I slighty creased it reapplying the protector when I was off axis. I can't see it when the unit is on in anycase.

Link to the Hori PSP screen protector:
http://www.ebgames.com/ebx/product/255993.asp

I thought I'd include a before and after pic of the job.

Before:


After:


The yellow in the bottom looks faded but I honestly can't tell a difference in person.