Those are two pretty different artists you mention. Conner Oberst is rather Alt/progressive and Ryan Adams is kind of alt-country-folk-rock combination, and is not nearly as critical and cynical with his take on America. Can you be more specific about what it is you connect between the two?
Based on my own take of those two...hmmm:
Josh Ritter (just introduced to him - brilliant songwriter)
Laurie Anderson's latest, Homeland
Wilco (seconded)
Elliot Smith (kind of down-key)
Jim White (he'll cover the South for you)
Sufijan Stevens (working on those midwest states)
Mark Kozelek (also on Sun Kil Moon & Red House Painters)
More classic takes on "America" that are less like the artist you mention:
Johnny Cash
Leonard Cohen
Dylan
