This has been discussed a lot. For omni directionals, search these forums for the following topics:
"antenna", "antennas" (sic), "antennae", "Antenna Specialists", "Fab-corp"

The last two are dealers. One of the people at Fab-corp is on these forums sometimes (Borg Man) and offers a discount to others on these forums. The people who have about one of the Fab-corp models rave about it.

A car body, because it contains a lot of metal will block an RF (radio frequency) signal. (A pretty obvious statement, when you think about it.) The "pringles can" and "coffee can" tend to be very directional. If you point them outside the car, say to the left, you might very easily miss an AP on your right. To wardrive well, you really need an external omni. BTW, don't point a directional at body parts (especially your eyes), or have a body part with about 20cm of the radiating (open) end of the directional. A directional antenna concentrates the radio energy. This stuff is low power, but it is still microwaves.

Cheers,
Thorn