Not sure if you can get them in the US easily, but in the UK - I got mine from
Wiggle, but I've got a Roox S4.2. It really is a lovely thing too.

http://www.roox.at/COMPONENTS/Roox_C..._Startpage.htm

you can see from the pic that the post has a hole drilled through that actual
tube, which a beefy metal rod passes through - then the twin bolts go through
that to the clamping part at the top. You may think that a hole drilled through
the post might weaken it - but internally the tube is thinner at the bottom but
thickens considerably toward the top.

Other nice feature are a practically scratchproof anodization which stops it
from getting marked one removal/insertion. But even better are the 1cm markings
along the length of the tube so you can see how far it is in the seat tube.
Useful if you usually have 2 'settings' - one for the uphill grinds, the other
for the downhill fun.

All in all a great seatpost, and with plenty of choice of diameters...

CandT