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Another option-- if it is available to you-- is a grocery store in an area with significant population of recent Eastern European immigrants. There was a Polish store in Ann Arbor, MI (until it closed last year) where the live yeast was available by weight (cut from a big block, of the same consistency as cake yeast).
Try a homebrewer's supply store, or ask at a local bakery if you can buy some of theirs off of them, or (even better/cheaper) you could make your own sourdough starter.
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