Functionally, there is no aftermarket navi unit as good as Toyota's. The ONLY touch-screen navi unit aftermarket is the Kenwood unit. Even then, Toyota's system is more user-friendly.
Alpine makes a popular model. However, it uses a remote to control the navi unit. Same for the Pioneer system.
I have no idea where to get Toyota's system besides the dealership. It's gonna cost you $$$$$$$$$$$$! Maybe try the local junkyard. Or Ebay...they have everything!
Garmin has just announced the Streetpilot 2610, this is a portable auto navigation system with touchscreen and voice navigation, they are set to be released on 8/25 i have pre-ordered one from www.onlinemarine.com.
This looks like the best alternative to the factory units and it is portable to other cars. Take a look.