Here's one way to build one, it's only a tutorial on how to build your own carport connecting to your house but it looks really nice, just as nice as anything premade.
http://thesims2.ea.com/exchange/story_detail.php?asset_id=53552 if this is the incorrect way to link I apologize, It came from ea's sim2 exchange, in their stories section. Also buntah has some nice canopies that make great driveway carports some are even come with lights, but I'm not positive where I found those. But I did see them being shown at Linda's Sims2 Blog. Alot of canopies can be used successfully as carports with the moveobjects cheat on. It will not block your cars from coming and going. Hope this gives you some decent options and maybe a few ideas of where to look next.