Autosport Catalog. I've ordered two car covers from them and love them. They are custom made for individual car applications. I got the NOAH cover, works great for indoor/outdoor use. Super soft and breathes, so if it gets wet it won't mildew underneath the cover. They are a bit pricey–think the one for the 370 was ~$250– but it is money well spent in my opinion.
AutoSport Catalog - NOAH Car Cover 800-953-0814