The Unitied States Geological Survey provides seamless downloads of any area of the globe from the 1 arc-second SRTM data set (~30m), as well as other datasets of higher precision but lesser coverage.
Ashundar.com hosts a lot of excellent artificial and real-world terrain files in a range of sizes (257×257 to 8193×8193), including a 3m DEM of Mt. St. Helens!