Originally the Indiana Jones-fedora was provided by the british company The Herbert Johnson Hat Shop.
The fedora varies quite a bit from movie to movie, and even from scene to scene.
A good fedora is the Stetson 'Temple' which is my favorite fedora http://www.levinehat.com/store/comersus_listCategoriesAndProducts.asp?idCategory=49
My fedora is a 50 year old Stetson, so it isn't quite old enough, but here it is... Yes, I know it is the wrong color!