It's a personal choice.
Yes you can buy extinguishers at Medards/Lowes..etc, but make sure you buy one that can be recharged. Also if your going to putting in your vehicle it pays to spend the extra money and buy one that doesn't have plastic components and comes with a vehicle mounting bracket. If a extinguisher discharges in your vehicle because it was rolling around and the pin fell out you will be hating life. Powder is so fine that it gets everywhere even if it's a short discharge.
Also I would recommend not wasting your time purchasing those little 2 1/2# ABC extinguishers, the discharge time is less than 10 seconds. Purchase a 5# ABC if you got the room.
Those Stop Fyre extinguishers Tikka mentioned are nice, I like that they are a filled with a gas and not powder but for $300 bucks I would rather buy a 5# cleanguard (gas - no powder) from a vendor in town that can recharge it on site rather than having to send it in to be recharged. Each their own though.