You could just grow your own with instructions on how to make sourdough starter.
Notes:
- it says unbleached, but you can use bleached flour with mashed up unwashed seed-less grapes the first day to start.
- boil the jar/lid and water you use to sterilize first . Let cool until you can easily handle with your bare hands or it will kill the yeast.
- it will take a week+ of daily feedings for it to be ideal starter. Once you have a healthy batch you can store it in the fridge and refeed it weekly/every time you use it.