New Mitel gear is available only through Mitel authorized dealers in the US. There is no wholesale distribution channel except for certain smaller platforms, i.e., the SX200/200ICP and telephone instruments sold through Sprint North Supply and Graybar Electric. If you think it's expensive, go price Nortel, Avaya and Siemens for comparable equipment. As expensive as it is I think you'll find Mitel to be best bang for the buck.
There is an active grey market of used equipment from several vendors, just beware that software licenses are often not transferrable so you wind up with a rogue unit that you cannot add features to without first purchasing the core S/W load.