If you’re looking for cheap sunglasses, the Galle Road is perhaps the best place. Right across from Majestic City there’s a stall which sells reliably cheap sunglasses (Rs. 200-500, depending on negotiation skills) which, while potentially bad for your eyes, often look good.
If you cross the crosswalk in front of Unity Plaza’s Vijitha Yapa, you’ll run into a guy selling belts and belt buckles. Go left and before New Majestic Hotel you’ll usually see a guy selling assorted plastic junk, including sunglasses. They usually have a reliable selection of Fay-Bans.
Another good spot for cheap shades is Galle Road, Wellawatte.