The Walhalla site is just a list. All of the download links are either AvSim or Flighsim.
While you're at Flightsim, go to the Search page and click on the Advanced Search link. Select FS2004 Senery, type in "landmark" in the top box, go to the bottom and sort by downloads then submit the search. You'll get some good stuff. Then go back and change the search term to "landmarks" (plural) and get even more!