After read this helpful thread, i try to seach"Pandora Battery " on google, and i have find some source which sell this battery,
one is gogamestuff.com, the product name is "Pandora Battery for Sony PSP", you can search on the google, you will find the site.
here is ther pictures:
It said"Using the magic memory stick and the magic PSP battery you can Unbrick& Downgrade any PSP easily."
Another site is uxcell.
Here is just what i find on google, any one have other source , please feel free to share with us.