I'm afraid that AutoCAD 2006 doesn't seem to be available any longer (but you can purchase 2011 as a download on the AutoCAD site). You may be able to find a copy on eBay. Is there any particular reason why you need the older version?
I'm assuming, of course, that you were talking about buying the software rather than enquiring where you could download a pirated copy (that would be liable to get you banned from the forum, so I'm sure that wasn't your intent). The company's web site is always the best place to look initially.