iGo tip number B07 RS catalog number 273.1937
iGo Auto 15 RS catalog number 273.1301

it's $30 US all together but is warrantied against frying your PSP. the tip is designed for a Dell Axim but the polarity, voltage, and size fits perfectly.

sorry if this is a repost. thought this would be a better alternative to hacking a cable.

oh, and if the RS sales person doesn't agree that it fits, have them look it up on their Answer's Online.

another idea is to buy the GBA cig lighter adapter and just use the piece that goes into the cig lighter. it accepts a standard USB end. if you have a charging USB cable for your PSP, this would work. RS 26.657 retails for $9.97