OK, if that's all you need, you can get a basic 1/4 wave VHF NMO mount antenna for a few bucks and it'll work well.
If you are on the fringes of coverage, you can try a half wave or 5/8th's wave and get a bit more gain. But you probably don't need that.
quarter-wave VHF 152-162 MHz
theantennafarm.com
That'll work well on VHF.
It'll also give you some performance on UHF (1/4 wave VHF is 3/4 wave on UHF)
If you want to play around with the VHF, UHF and 700/800MHz capabilities of your scanner, you can try one of these. Both are good antennas and I've used them myself.
150/450/700/800 Antenna, Gain (dB) Unity, 100 Watts, Whip Length 16.5"
theantennafarm.com
or
EM-M43002 Tri-Band IP67 Rated Roof Mount Antenna, 150-162/450-490/763-870 MHz
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You'll also need an NMO mount. And then it depends on how you want to mount it.
Permanent mount is best, but not everyone wants to drill holes in their car.
You can do a magnetic mount:
Magnet Mount NMO with 12' RG-58 and BNC Male connector installed
theantennafarm.com