Glad to hear that your project is still going on
I bought most of my electronic stuff here: http://www.conrad.com/
, although I could have easily bought at a local store. The part number of the voltage converter is: 19100 (just enter it in the search field)
Extending the usb was pretty easy, since I just bought this genderchanger: Neutrik NAUSB-W-B. So you simply plug in a standard mini-usb cable and connect that to the dingoo.
I bought my converter (and all other arcade parts) at http://www.arcadeshop.de
For preventing the usb-connection "charge" the converter, I applied two protection diodes. I don't exactly know which one it was, but it could be a 1 N 4001 I=1A U=50V.
You can also order that one from conrad.com.
As for the soldering I haven't made any guides, but it's pretty straight forward. Just google for some joypad hacking guides, it's almost the same deal. All buttons are connected to the same ground, which makes it pretty easy.