I have never heard of a steampunk convention, but here it is...and they have vendor spaces:
The best is yet to bead!
There are a couple others also. Here in Metro Detroit at Henry Ford Museum and/or Greenfield Village is Maker Faire and the World Steampunk Expo. The World Steam Expo I know has vendors and may still have spaces but their deadline is April 1, 2011 http://www.worldsteamexpo.com/ The link for Maker Faire for Detroit (they have them other places) is makerfaire.com/.../2011