STANLEY Cafe To Go Travel Mug 20 oz | Leakproof Two-Part Drink Through Lid | Tumbler Cup for Coffee, Tea and Drinks | Insulated Stainless Steel Coffee Mug | BPA-Free | Frost

This is the only travel mug that I have found with a positive seal screw down top over the sipper. After you have used it, you can screw the top on, throw it in your bag, and it won't leak. Travel mugs with sliding or press down seals always wear and will eventually leak. The seal rings on this one will eventually wear too, but you can just replace them with food safe silicone O-rings which are readily available on the Internet. This mug comes in various sizes. This is the largest one (16oz = 454 ml), which will still fit under the coffee machines in Waitrose and other supermarkets. If you are in the UK and want to buy one, it's much cheaper on Amazon.com even after adding shipping chanrge and VAT.