Best Substitute for Yeast in Bread Machines?

The one persistent shortage I'm seeing in my part of L.A. is of baking yeast, and the online stores seem out, too.

I've seen various online recommendations for how to substitute other leavening agents for yeast in various bread recipes, but I'd love to hear any experience you folks have, especially when using bread machines—and other readers might find it useful, too.

I realize the result won't be identical to that with yeast, but I just wanted to get a sense of what's worth trying.