Can't help you on the standing desks, but;

Have you considered using an exercise ball instead of a chair? Am I correct in assuming that you just want a standing desk for the health benefits? If so, exercise balls are pretty good for you as well (at least better than a chair). They are a lot cheaper than standing desks.

http://www.gearfire.net/10-reasons-to-use-an-exercise-ball-a...