Warning : this article is a stub. Please help by improving/commenting it!
You can use the free Tiger maps for US.
Quick howto :
The conversion script converts tiger to osm v4 (where navit uses osm v5) but hopefully there is a script which can convert osm v4 to osm v5...
So the procedure should be something like :
Get your datas from tiger. You can get the tiger to osm tools here : http://sr71.net/~dave/osm/tiger/
Now convert the resulting datas to osm v5 (you can get the tool here : http://trac.openstreetmap.org/browser/applications/utils/conv05 )
Now, convert the datas to navit binfile :
./osm2navit mymap.bin < mymap.osm