Get tea stains out of a stainless steel tumbler?
A wet Mr. Clean "Magic Eraser" sponge will wipe them right off. Same with ceramic tea or coffee cups. If you don't already have a few, just make a paste of baking soda and water and that will do the trick too.
Baking soda and water, just let it soak a few hours, or for faster results make a paste and a sponge.
Put a teaspoon of liquid dish WASHER not dish soap in the tumbler and fill with hot water. Let sit overnight. Wash in the morning. It should be clean.
For my mugs I use a solution of Vanish clothes stain remover. Add a few drops of washing up liquid with the Vanish powder, fill with hot water to mix and leave overnight, then just rinse well.