I wish you could "watch" the AH for certain cars, and get a notification when one went up for sale. I missed out on the Twin Mill, and I've looked multiple times for a Crown Vic., but it's such a pain to manually re-search every time.
You can win one if you get high enough in the Team Adventure leagues, other than that you’ll just have to get lucky on the auction house. I found mine there for 600k iirc.