Do you know where in Polk county did he die? Or where he is buried. Most local libraries have their local newspapers on microfilm, so that's usually where to look for an obituary.
Findagrave has a William P. Warnick listed in the Polk Cemetery, so try obits at the Polk Library. You could call and ask before making a trip if you want. Stromsburg has some other newspapers on microfilm, too, but I'm not sure which ones. Hope that helps.