I've brought two cars from the US over the last few years. Was looking at doing a few more before the economy tanked. It's pretty straightforward. Check out this website for more info and links to resources etc.:
http://www.carswithoutborders.com/
I don't remember how exactly I found my wife's Volvo. Might have been some specific car sales website. My Audi was found on craigslist in the SF Bay area.