is sold during lunch (and brunch?) service at Bar Tartine. They have four kinds of enormous rounds for $8 including country, seeded, and an oatmeal bread, along with a dense half rye for $5. You can even talk to the baker.
Bring a bag though, or they'll charge you a fee. Or you'll have to walk down the street with a child-sized bread in your arms.
Have lunch while you're at it. The sandwiches are great, and they couldn't be nicer.