WAGDA hosts a limited amount of Lidar data for the Mount Saint Helens area. These data were generously provided by the USGS and are made available here. The main resource for Lidar data around the Puget Sound is the Puget Sound Lidar Consortium (free registration required). They have Lidar coverage for most of the Puget Sound lowlands available. For general information about Lidar, see the Puget Sound Lidar Consortium's page here.