That type of promotion lends itself to a range or platforms, Social Media especially, but even a platform like eBay. I would check if you cal list sunglasses under a classified option and if you can, do up a promo on the listing advertising your glasses along with the promo give away. Try and link it to a newsletter subscription perhaps.
Join our monthly newsletter and go into the draw to win a FREE pair of sunglasses. That way you get the added bonus of harvesting email address that you can then market to directly and it gives you a base to then develop and market your own website to.
Whatever you choose to do, try and make it count. You don't want to just give away a pair of sunglasses and get no return at all out of it!