Psionic used to have some free skyboxes to download, but they aren't available anymore. Pity. The actual sky textures are easy enough to generate (below is a tutorial on how to create the images using Terragen,) but his samples also had code.
http://developer.valvesoftware.com/wiki/Creating_a_2D_skybox_with_Terragen
http://developer.valvesoftware.com/wiki/Creating_a_2D_skybox_with_Terragen