I am using a PRS-600 sony ebook reader which is not available in Thailand so it is almost impossible to get a charger for it. However I have read on various forums that the Sony PSP charger is compatible with this device. Where can I get a cheap PSP charger in Bangkok?

I would look around the shops where they sell PSPs, although this may not be the cheapest option. If you are more adventurous then maybe Spahan Lek, Ban Mo, Khlong Thom.

I looked at one PSP charger: Sony PSP-384 AC Charger, 5 V, 1500 mA. This is a "travel-type" charger.