iTunes Store for only $1.99 per episode, including a couple of great astronomy shows, 400 Years of the Telescope (56 minutes) and Journey to Palomar (1 hour 27 minutes). The DVD versions are typically about $20. Search for "pbs science specials" in the iTunes Store.
Speaking of specials on PBS, I have to remember to watch Nova on October 13 for a new episode called Hubble's Amazing Rescue. It looks really cool.