The Decepticon Matrix does not exist.
Shockwave's continuous experimentation eventually led him to discover the true location of the Vector Sigma supercomputer. By kidnapping Alpha Trion to gain access to the computer, Shockwave hoped to create a Decepticon Matrix of Leadership to further enhance his iron grip over what remained of Cybertron. His attempt was interrupted by the timely arrival of the Wreckers, who rescued Alpha Trion shortly before a massive Sharkticon flotilla attacked Cybertron.
Similarly, Megatron was promised a Decepticon Matrix of his own by the Quintessons in exchange for cooperating with their peace treaty with the Autobots. After the Quintessons' inevitable betrayal, both Megatron and Optimus Prime fought their way to the Quintessons' inner sanctum where they found both the treacherous Quintesson medic Xeros and the Matrix replica the Quintessons had been using to manipulate Prime's Matrix. Believing it to be the Decepticon Matrix, Megatron inserted it into his chest despite Optimus's and Xeros's warnings. However, it turned out to be actually the Sharkticon Matrix and gave Megatron a new Sharkticon-based body and complete control over all Sharkticons.