I also discovered a great visor on my last trip. It had a nice big brim, so it shaded my face really well. It was made out of a really nice material - it looks like straw, but it's really some kind of cloth.
What was also great was that the band attached together with Velcro in the back. When I wasn't using it (when indoors), I just unhooked the velcro pieces and looped it through the strap on my bag or fanny pack.
Didn't give me "hat hair" or make me sweat at all. I can't remember where I bought it, though -- I think it was either Target or Kmart last summer. Only cost about $6.