Hi Lindsay, if the garage is attached to the house then, yes you are required to have one. Car exhaust is another source of potential carbon monoxide fumes in addition to gas appliances. There are battery operated models that you can mount to the wall as well as plug in models that just plug into a standard outlet. Generally you'll place a detector in the hallway near the bedrooms.
Yes, it is still required one per floor. They should be located near the bedrooms. The reason is that because of attached garage and vehicles carbon monoxide can still get into your home. This law is for your safety. Hope this helps!