How about 'free' chargers?
I use a Sony charger that I got for free (well it cost me about a $1.50 for a new power connector at RS).
The chargers used on old Sony Clie PDAs are the same voltage/amps as the PSP/PRS-505, but have a USB-like connector on the end. Simply snip the existing connector and replace with one that fits the PRS-505. Ask around or look for old Clie chargers which have outlived the PDAs.