www.mp3car.com . They have a huge forum that covers everything form hardware [custom and off the shelf] to software [custom and off the shelf]. They also have a store.
www.mini-box.com - Store.
Those are the main places I went to, but there are many other places on the net. You can google "mini itx" or "car pc" and come up with tons of sites.
Both are good resources. I had a puter in my WRX. I still have the puter, but now that I have the IS350, I dont need it in my car anymore. I ended up spending about 800 for the puter [GPS, wireless and everything but the monitor], and you can get a 7 inch touchscreen for about 250.
let me know if you have any questions.
cg