I've been using the Radio Shack charger w/interchangeable tips (6V, 800 ma) for years with Sony readers. It comes with one free tip and the PRS Readers use Size B. Works very well - costs $19, available almost everywhere.

Also, it' a myth to let the batteries run down - that was true for NiCads but letting lithium batteries run down will damage them - you can check this out by Google