I've always had great luck with Big Sky Car Covers (www.bigskycarcovers.com). I use the Noah fabric for my outdoor covers - nice because it breathes, rejects almost everything from outside and rolls up reasonably compactly. Big Sky generally drop ships from CoverCraft (they make almost everyone's covers but won't sell direct). Cost & delivery time depend on what CoverCraft has in stock.

It doesn't show on their web site yet, but I would assume they have a pattern for the Gallardo by now but who knows. You probably will need to call rather than check just the web site.