I recommend you start with an accessory item known as a "retainer strap" - a device designed to insure that vision glasses don't go swimming, often available for under $10. There are even models with flotation that will float many models of sunglasses. I have fished with expensive sunglasses for 30 years with such a device, and have not once lost a pair overboard.
^^what he said!! Costa's have been the best sunglasses I've ever owned. Have used at least 8 other brands and for fishing specifically I always grab my Costas. If you find a pair that fits properly you might not need a retaining strap. I use the Blackfin model and it fits snug even on my big head.
Also what the other guy said try that steepandcheap website. I got a 580 glass lens pair of Costa's for like $150 on there one time!