I bought one off ebay, when you search you will find some from @£10 - £19.
I bought the more expensive one, which if I remember was free delivery, this has all the electric bit that transform from 12v to your input at 19v @2.1amps (40w).
I left mine on for a number of hours and the bike still started, great bit of kit with long lead.
Won't that be quite power hungry as you'll need power to convert it up from 12v DC to AC then more power from the netbooks convertor back down to whatever it wants?