The starter that I started back in the 1980s used plain yogurt. The information back then was that other dairy products were too pasteurized to have the correct enzyme or bacteria needed for sourdough.
Sourdough comes from a certain type of wild yeast that can live with the bacillus from the milk. The bacillus kills the types of wild yeast that will not make the dough sour.
Look for instructions that use either unpasteurized milk or plain yogurt.
Just as a note, a freshly made starter will not usually punch up dough well enough to use without bakers yeast for about 6 months.
Thanks for the info Ron!