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Check out the MyBot Tutorial and specifically its xacro file :

<macro name="cylinder_inertia" params="m r h">
      <inertia  ixx="${m*(3*r*r+h*h)/12}" ixy = "0" ixz = "0"
                  iyy="${m*(3*r*r+h*h)/12}" iyz = "0"
                  izz="${m*r*r/2}" /> 
      </macro>

<macro name="box_inertia" params="m x y z">
        <inertia  ixx="${m*(y*y+z*z)/12}" ixy = "0" ixz = "0"
                  iyy="${m*(x*x+z*z)/12}" iyz = "0"
                  izz="${m*(x*x+z*z)/12}" /> 
</macro>

<macro name="sphere_inertia" params="m r">
        <inertia  ixx="${2*m*r*r/5}" ixy = "0" ixz = "0"
                  iyy="${2*m*r*r/5}" iyz = "0"
                  izz="${2*m*r*r/5}" /> 
</macro>

These macroses are calculations for basic shapes and could be easily pulled into your project. But if you need inertias for some custom shapes, then you should look into some 3D modeling SW, they have capability to calculate necessary matrices for you.