Alot of laptops DON'T HAVE car adapters, because of the higher-than-12V-input. Most Sony's fall into this catagory - pisses me off to NO END.
Alot of the reason behind this, is that alot of the devices inside today's high powered laptops, require 12V, so the batteries need to be 14-16V to supply that, and the DC inputs to charge/run off wallwarts need to be considerdably greater still (16-21V typically).
I've got a Sony GRT-100, great machine - desktop power in a laptop size, 16.1" screen. Works great for me in my day-job, but the internal battery only runs it 35 minutes, and there ain't nuttin' out there to hook up externally that will work (tried an electrovaya powerpad, valence doesn't even come close). You "could" try running it off a 12V SLA and an inverter, but I don't know how efficient this is going to be.
Let us know if you find a 12V Car adapter tho', I'd love to try and find something to run my lappy too (or take a beating, dump it, and by a CENTRINO that I CAN run for extended time in the field)...
Rick