Any of these lids from Yeti is a great replacement, especially if you've misplaced yours.  Plus, extras are always handy to have should you need it! Dishwasher-safe, shatter-resistant, and splash-resistant. Comes in a variety of sizes to fit your Tumbler.

The MagSlider lid also comes in a variety of sizes (for your Yeti Mug, Tumbler, Lowball, and Stackable Pints). Its smooth magnet allows for easy sliding to protect your drink and prevent heat and cold from escaping. Easily keep your drink from spilling or splashing.