Output impedance of voltage divider?

The output impedance is the equivalent of the two resistors in parallel.

If you are lazy, you can approximate this by taking just the lower-valued one, in this case the 5k.

Wouter has the right answer, here is the background why.

A voltage source in an ideal sense has zero output resistance. In terms of small signal analysis, this can be replaced by a Ground, grounds have zero resistance and in that case your drawing reverts to:

^{simulate this circuit – Schematic created using CircuitLab}