Cris - The three main vendors for legitimate preparation tests are Transcender, Self Test Software and MeasureUp. Transcender and Self Test are both owned by Kaplan so really they are the same company but sell the products under different brand names. None of these have released a SQL Server 2012 preparation test yet.
Microsoft's answer to this conundrum has been to offer the Second Shot promotion so that you can retake the test for free if you fail it the first time, effectively giving you to real test as a practice test. You can find more details about the Second Shot on Microsoft's web site.
Whatever you decide to do, don't under any circumstances use a "braindump" or "actual exam" web site as these materials are cheating.