Use the Cleverscope Frequency Response Analyser to measure the input impedance of a power supply with the CS1070 Power Amplifier.
It can manage up to 1A output current over a voltage range of -18V to +30V over a frequency range of DC - 52 MHz. This is sufficient to directly measure many power supplies. If this is not enough, you can inject across a low value series resistor (eg 0.1 ohm). This video shows you how to connect up a CS328A-FRA and CS1070 to measure the Input impedance of an LTC3589 over the frequency range 200 Hz - 10 MHz.