If you don't need the latest style steepandcheap is a great place to buy from. I bought an extra pair of Costas 580P to keep in the boat for $58. Regular price was $198. If bought from these folks in the past and glasses are the real deal and the shipping is fast. Also a good place to buy raingear from but you have to keep an eye out for the sales.
http://www.steepandcheap.com/gear-ca...d#?brand=Costa