If you don't want to buy a dehumidifier you could try to find some silica gel. I don't know how well it would work on a large scale grow room, but companies put silica gel in their products' boxes to remove moisture.
Also (something I would do), is buy a dehumidifer from Target or Wal-Mart, use it, and then return it after your done. I know both of those places have a 90 day return policy and don't ask you why you're returning it.
Good luck.