denmarks, grooks asks a good question: Where are you located? The availability of parts, and the shipping/tax/duty/etc., may depend somewhat on where you're ordering from.
My quick Google search turned up a couple of options.
Here is one site that sells a USB to PSP charge cable.
This one is cheaper, and has a combination of data sync and charge tips. There are a lot more out there.
I found another
cable that is even cheaper on eBay. The seller is in Hong Kong.