I have brought snakes home from the expo before. The breeder normally will give you the animal in something to transport them in. For hatchlings and small balls it will be a deli cup usuallly. For sub-adults to adults i have seen pillow cases, boxes, and foam coolers all with ventilation of course. As LTG stated just keep the car warm.Also with multiple purchases I bring a plastic tote just to keep them all together in the car.
**If the vendor does not give you something to transport the animal in i would not buy from them. I have never seen this done even with feeder rats.**