I use PSP GRIP With Built-in 6 Hour Rechargeable Battery, I ordered at Amazon.com
The unit was obviously designed for the Fat PSP, to make it fit my Slim I added two simple modifications:
1 I folded and glued togeather pasteboard inserts to fit in the lower hooks to better hold my PSP

2 on the back of the grip I glued the sides in place so they wouldn't fold when I was playing
I love the unit, it makes it much more comfortable to hold
