I bought one for the MINI Cooper I used to own years back, it was handmade by a company in Littlehampton who do them for a variety of cars (all tailored to fit the model not just generic ones) and was much cheaper than the genuine offerings at the time.
Mine was outdoors but fantastic quality.
Seems they have moved to Worthing now -
http://www.cover-zone.com/Edit: they offer free covers for car models they haven't got in their catalogue - get in there first!