People drive around during Songkran in pick-up trucks and splash water on all and soundry. As the pick-up is the prefered type of vehicle there are plenty of "road warriors" everywhere. Typically, there will be a barrel filled with water in the back. And often a block of ice is added, so that even the already wet will notice if they get splashed.
When the water is gone refills are available from anyone by the road who has a hose. Note the green barrel in the right of this picture with a hose. Industrious vendors also sells ice blocks by the roadside.