Years ago I tried making my own starter by setting out a bowl of flour and water and hoping some wild yeast would alight. I never had much luck, so this time I bought a little ball of live starter. Mail order! From Breadtopia. So far so good.
This comment on Amazon sold me on Breadtopia:
The wet dough sourdough product is excellent. Upon feeding and expanding it woke up very quickly and leavened loafs extremely well. Lots of classic sourdough taste and lots of gas production. But as a university microbiologist, I can’t just try out a sourdough starter. I had to take it to the lab and find out what’s in it. I am happy to report that the starter contains precisely what it should: one species of yeast and one species of lactic acid bacteria. It is a complex task to definitively identify them as the authentic Saccharomyces exiguus and Lactobacillus sanfranciscensis, but everything is consistent so far with them being derived from authentic San Francisco sourdough. In particular, as one subcultures them repeatedly, the levels of each stay pretty consistent. This is a hallmark of an authentic sourdough starter pair of microbes. I highly recommend this product.