Are all flags and maps considered to be "public domain" for retail use?

I want to create at least 2 more stores. Out of respect for Fluther I would like to refer you to their product site, although the question is about mine. You get the idea.

http://www.zazzle.com/fluther

I wish to use public domain graphics to create products. With that being said, what would be acceptable use? I was told I could use anything as long as it’s not been altered by an artist. Is that true? Are there more well known graphics that are public domain that I can look in to?

Your wisdom in these matters would be appreciated.