I went to the New Balance store. They are the only athletic shoe that fits me reliably (they make narrow and wide widths, as well as regular).
I really liked the person who helped me. He ended up recommending a running shoe, because it was more flexible than most walking shoes and less likely to aggravate some of my foot issues.
You might also try Road Runner Sports. They have a store in Falls Church. I haven't been in there, but they are supposed to have a machine for measuring your foot and stride. Sounds cool to me.