[quote name='Hybrid5006']I suggest ebay... should be around $20 shipped...[/quote]
Don't trust any of that knockoff crap on eBay. I would advise against buying batteries or memory cards from there as most of it is cheap knockoff shit!
If you have any Sears, K-Mart, or TRU stores near you check them for PSP batteries. In my area I bought the regular strength phat battery for $7, the stamina battery for $10 (at K-Mart), and I've read reports of CAGs finding them as cheap as $4 on clearance.