24th & Hilyard: The massive outside community bulletin boards (one by the door & two by the bike racks) at
Sundance Natural Foods, are so popular that Sundance needs to take everything down every week. In anycase, you have to go there weekly, or your poster will get covered up. You can post one on each of the three boards (the ones on the corner aren't purged quite so often).
Inside, there are bulletin boards on the refrigerator aisle by the back. These require special permission ... but, being a non-profit, and an educational organization, we have one there already. Worth checking back.
If you have a large, specific event poster, especially a non-profit event, you can ask the people at the desk if they could put it either on the end of the aisle by checkout, or in the window.
If you have a coupon for a non-profit, or a specific event, they can put them at the checkout itself. But they won't do this very often.