You can't find any merchants in the game selling Glass, the crafting ingredient itself. However, you'll find the merchant NPC called Vennor Skinflint selling Glass Flasks in exchange for Silver Coins in both Exiled Lands and the Isle of Siptah maps instead. He asks you for one Silver Coin per 10 Glass Flasks each time. Find this merchant by Sepermeru City in the Exiled Lands and at the Camp of the Castaways in the Isle of Siptah DLC map.