Some possible leads:
I have found a couple of websites that may get you what you want, and a couple that are extra fun:
Since I can't guarantee that I have found the poster, I would contact the Center for the Study of Political Graphics and ask if they have any Yvonne Wanrow posters in their holdings.
At the University of New Mexico, there is the Inventory of the Robert E. Robideau American Indian Movement Papers, 1975-1994. Yvonne Wanrow's name shows up twice in the contents list of the finding aid (click the purple links with the red arrows to get there). It's possible that the poster MAY be in "AIM Materials, 1979-1993" which is box 22 of the collection- Yvonne Wanrow is in folder 11. You'll have to contact them to verify anything.
The University of Delaware Library has the Neala Schleuning
Poster Collection which in folder 6 has a poster with this description: Native American Female Political Prisoners (Joanna Le Deux, Anna Mae Aquash, Yvonne Wanrow, Kamook Banks).
Native American Solidarity Committee. Produced by Workin’ On It Lesbian Press, Seattle, WA, n.d.
The California State Library has the "Gildersleeve Berkeley social protest collection". Search the catalog by keyword, and NOT by subject, for Gildersleeve and you'll find the collection. You'll have to contact them once again for verification if there's a poster in the holdings, but Wanrow is mentioned.
The Online Archive of California has holdings of the Royal Chicano Air Force, a poster collective. While not the image you are looking for, they still have this . You can also do more searches in the Online Archive of California and find related material by the RCAF and other stuff from California.
The Yanker Poster Collection of the Library of Congress also has this list. Go down to Wanrow, Yvonne and see the posters that they have.
The Evergreen State College Archives also has an inventory of posters and artwork. Poster 108 mentions Yvonne Wanrow.
I hope this is the start of some interesting searches...
And: another colleague at RadRef found a set of posters from the Chicago Women's Graphics Collective which was an explicitly feminist collective that lasted from 1970-1983. Yvonne Wanrow is not in the holdings, but the posters may interest you.
political poster resources
Aha! Stumbled across this query while cataloging this very poster, an example of unintended good consequences of networks such as RR.
In addition to the above suggestions, one can also contact me, I'm an active scholar on these materials and can often help. Look me up as Docs Populi, Documents for the Public. I have many examples and further links. You'll find that in many of the above collections the materials are poorly or incompletely cataloged, and rarely include good images for reference. The specific poster you mention was independently produced in 1976, designed by K. Sjoholm.