The antenna is the most important part of your setup and getting a good one pays off in the long run.
You can't go wrong with these, and I've used both extensively on multiband mobile transceivers:
Running this one on my work truck:
EM Wave EM-M43002 Tri-Band 150-162/450-490/763-870 MHz Mobile Antenna - 17.7 in - NMO - Spring Base
theantennafarm.com
Running this one on my personal truck:
Larsen NMO150/450/758 Tri-band Mobile Antenna covers 150-174, 430-520 & 750-870, 100 Watts, Length 16.5 in, NMO Mount
theantennafarm.com
If you don't need the spring base and want to save some money, this has received some excellent reviews and will likely outlast your car:
Larsen NMO150/450/800 Tri-Band 150-165, 450-470 & 806-940 MHz Mobile Two Way Radio Antenna Scanner
theantennafarm.com
If you need a magnetic mount, pair it with a decent quality NMO mag mount with the correct connector to match your radio:
Heavy Duty, Low Profile Magnetic Mount. For use with NMO style antenna. 12' RG-58/U Cable, BNC male connector Installed.
theantennafarm.com