Almost all sourdough recipes I've seen start with a purchased yeast in the very first batch. The yeast is just propagated in the starter, instead of starting at zero in the next batch. Of course each successive batch would have less of the original ingredients, but there would be some in the first few batches.
There are lots of sources of organic yeast, in bulk and consumer varieties. These do not use the emulsifiers and anti-foam agents (although some use a natural oil for anti-foam, which is fine.) You could just use the organic yeast instead of bothering with sourdough, which takes up a lot of refrigerator space and is fairly high maintenance.
Check out
Shipton Mills, or
Bob's Red Mill.You might also be able to find some organic yeast locally at an organic food store.
Single empty-nester in Middle TN, sometimes with a friend or grandchild on board