I've been wanting to try this for some time. Using a commercial yogurt as a starter is fine but the subsequent generations are much weaker than the original. Ditto for the powdered probiotics. I rarely get more than a generation or two from either.
There's very little on the 'net about your own starter (as compared to creating your own sourdough starter). I came across 2 options: the chili pepper stems and ant eggs. I figured the ant eggs would be difficult to differentiate from dirt, so pepper stems it was.
I did the usual heat milk to 180 and cool to the incubation range. Then I added the stems and incubated for 24 hours.
The results was a very firm yogurt that didn't pour. Usually when the yogurt is still warm, it will pour.
I'm refrigerating it now and will taste it once it's cold. It smells fine and looks good.
If the rather meagre postings I have read about this are true, this should produce a mother starter.