King Arthur's current starter is the best by far that I have used
over 30-plus years of bread baking, but you seem to discount it by
the "other than . . ." in your question. There are many powdered
starters on the market, including Gold Rush purportedly made in
California. There are a plethora of instructions on making your own
starter; try Googling "sourdough starter" or check cookbooks that
focus on bread. The easiest way to get starter is to request some
from anyone who makes sourdough bread. I hate to throw away anything,
so I much prefer giving away the starter that I otherwise discard
when I feed the starter. Unfortunately, I live on the opposite side
of the country or I would be pleased to give some to you. I know from
experience that I can't mail it to you!
Ellen