Gadget shop ThinkGeek has the officially licensed Indiana Jones hat on sale. You can be Indiana Jones for $99.99.
This is a fur felt fedora that you see in the Indiana Jones movies on close-ups of Harrison Ford. It's a hand made and features a leather sweatband and satin lining both emblazoned with Indiana Jones in the inside.
There's also a little Indiana Jones charm on the hatband (which is easily removed for a more movie-accurate look).
The new Indiana Jones 4 (Kingdom of the Crystal Skull) is in Theaters on May 22nd. So, if you order the hat now, you should be all set for the movies.
More details on ThinkGeek.