The pet shop or breeder will normally give you a container to take your rat home in. This is usually a cardboard box with airholes but rats have a habit of chewing their way out of these and so they are not ideal - particularly if you have a long journey home.
Therefore it is best to go prepared and take a more suitable container with you. This can either be a small plastic carrying box designed for the transportation of small animals sold in pet shops or any suitable sized plastic tub (such as an ice cream tub) punched with holes.
Place a handful of woodshavings or some torn up tissue paper in the container so that the rat will be comfortable and a handful of food.
If you already have a cage and it is easy enough to transport then you can take this with you to go and collect your rat. However, do not fix a water bottle to the cage as the motion of the car, walking, etc will cause the bottle to drip and soak the cage. If you have a long journey provide the rat instead with a piece of cucumber from which it will gain some moisture during the journey. | 
