These EXT connectors themselves can be purchased from Link's ithic.com or dragonbox.de, the guide is old as I don't think Wizardstan has any more of these connectors. They're also a bit pricey in the shops, but they are rare since they've been discontinued from most part suppliers or have a minimum order of 1000 or more.
These connectors are pretty tough to solder leads to, but some people have had good luck doing so.. Someone made a tiny PC board (TinyBoB) that made it easier to solder wires to.. but he dropped from the community a some time ago..
Edit: also more specific information on the voltage of the UART/GPIOs on this connector are in MWeston's Pandora HackGuide