Glad you posted that as im about to buy a pair from FleaBay.Legit. Steep and Cheap is a reputable company, I think owned by the same people that own Sierra Trading Post.
eBay is another great place to buy them. Much like you find deeply discounted reels on eBay, you'll find deeply discounted Costas. I'm sure it's a workaround for MAP pricing. Dealers can't sell them at their stores for less than MAP pricing, so they make an eBay store, sell them under a different name, and ship them from a location that isn't tied to their "official" business. I imagine it'd be relatively easy for the manufacturers to track them down, but maybe not worth it. If you go the eBay route, just be sure to buy from a seller with a high feedback rating. I've bought 4 pairs this way.
There are ways to tell if they're legit when you get them, as well, though that's probably not as satisfactory. For example, (on glass lenses at least) the "580" watermark is only visible from the front of the lens but not from the back. Knockoffs will show a watermark when viewed from the back.