How can I get.tea stains out of a stainless steel cup

http://tipnut.com/how-to-remove-coffee-tea-stains-from-mugs/

Have you tried Barkeepers friend?

Im impressed by all the cleaning problems it solves, it may resolve your tea stains.

”Trusted by homeowners, hobbyists, musicians, and professional cleaners worldwide, Bar Keepers Friend superior hard surface cleansers tackle rust, mineral deposits, baked-on food, and other tough stains with gentle, bleach-free formulations. Available wherever household cleaning products are sold, Bar Keepers Friend makes industrial-strength cleaning safe, quick, and easy.”

https://www.barkeepersfriend.com/

Bar Keepers Friend is the only thing I use on stainless steel, nickle, or chrome. You can buy on amazon if your market doesn't carry it.