You can make your own easy. I think King Arthur has some recipes, but the essence is: out flour and water in a jar. Add water and flour daily. Stir. Repeat. After like a week you’ll start getting some faction (bubbles). I made a levain with mine when it was your and feeding part back to the mother really amped it up.

There’s birthing fancy to these, just time. The yeast comes from natural yeast in the local air.