Real life output impedance of a Unity-Gain OpAmp Buffer circuit
You don't need to know the output impedance of the op amp -- You need to know the output impedance of the noninverting amplifier as a block. This can be complicated because the output is in the feedback loop.

It can be demonstrated (http://masteringelectronicsdesign.com/the-non-inverting-amplifier-output-resistance/) that for an op-amp with output resistance \$R_o\$, that the effective output resistance of the non-inverting amplifier, \$R_oout\$, is
\$R_{out} = R_o * \frac{ A_{CL}} {A_{OL}}\$, 

where CL and OL refer to open loop and closed loop gains.  With a closed loop gain of 1 and an open loop gain in the hundreds of thousands, the effective output impedance is quite small (regardless of the op amp!!)