I just went through this myself..
After some research, both here in the archives, and online/phone calls, I figured out that Covercraft is the main supplier, and perhaps the only one in the U.S., for a custom cover for the X. They supply the covers to California Car cover, and most of the other sellers.
Here are the different series of covers:
http://www.covercraft.com/carcovers/custom_car_covers.html
The Evolution and the WeatherShield HP seem to be the most appropriate covers for an outdoor parked X in most climates, the WeatherShield HP being heavier duty and more expensive.
I spoke to Covercraft directly and the cut code for the X cover WITH mirror pockets is C10056PG, as you will find verified here in the archives.
But one might infer from the archive posts that C10056PG is the model of a specific cover. It is not. It is the CUT CODE. That cut code can be used for any series of cover, for a cut that will fit the X precisely. They are built to order by Covercraft when the order is placed by the vendor.
You can get other covers built by Covercraft with their materials as well that will fit the X, but not as precisely and without the mirror pockets, for less money. I believe one of them is cut code CA24TK.
Once you know which series you want, I would shop around a bit if money is a concern.
Here's a list of seller's from Covercraft's website:
http://www.covercraft.com/app/buy_online.php?cat=10
Keep in mind there are other Covercraft sellers that are not listed on the Covercraft page. It is not a comprehensive list. Google is your friend.
I just bought the custom (C10056PG cut) WeatherShield HP for a little over $200 with a little tenacious searching; most places have it for $300+. Will report back on it once I get it. I wouldn't spend more than $150 for the Evolution version (usually $200+).
Covercraft also has a closeout sale on their older (less advanced) version of the WeatherShield material custom covers ordered and shipped directly from them for $170+. If that interests you, call them and see if they're still doing it and can cut one for the X. Here's the info:
http://www.covercraft.com/cover_sale.html
Lastly, If it doesn't matter that much to you, you can certainly find an "okay" cover on Amazon or Ebay or at an auto store that will generically fit a small car for $50-$100. A few of those have been mentioned here in the archives. But I don't know that I'd feel comfortable trusting one of those to protect the X through extended periods of harsh rain, sun, or snow. For short periods of weather, or light weather milder climates, perhaps it would be fine.
Happy hunting.