I just built one a couple weeks ago. What we used is 5½ round post from the local farm store. We had a mini X and a track hoe to push then in the bottom. So from so good on it. Heres a link to how I did them. The last page or so is where I showed it.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=504716#Post504716

The people who say I can't do it can just sit the @^#% down and watch me. Friends call me Rusto I also subscribe to pond boss mag. http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=504716#Post504716