The closest town to Rincon de la Vieja is Liberia, on the Panamericana Freeway. In this town you will find plenty of places to sleep. Because Liberia is not a tourist destination, I do not consider it necessary to book your rooms ahead. When you get to Liberia, ask someone for downtown Liberia. There you will find a good range of accommodation.
In the town of Liberia you will also find organized tours to Rincon de la Vieja. Ask your hotel for more information.