Zappos

We just realized the other day that Zappos carries eyewear. This has been a great source in the past for footwear, especially because of their free overnight shipping and returns, but now they're a great place to look for sunglasses too. Here's the link to their eyewear page: http://www.zappos.com/eyewear.htm?ref=shopfor

We found a nice pair of this past summer's Marc by Marc Jacobs sunglasses, so its definitely a decent source.