https://www.amazon.com/Primula-Press-Iced-Tumbler-Black/dp/B0765V7CHY/ref=sr_1_5?keywords=iced+tea+cup&qid=1554735918&s=home-garden&sr=1-5 I have one of these. I use it to make hot herbal tea into iced tea. Fill the inner cup with ice, then put hot water and a tea bag/looseleaf herbal tea up to the line in the outer cup. Let it steep for a few minutes, then press the inner cup into the outer cup to ice the tea. It works wonderfully and I can have fresh fruit tea infusions on the go anywhere I can get ice and hot water. You can also get one of those pill boxes with all the different day spots in it and use it to carry a variety of teas with you and one or two of those 'days' can be filled with stevia or another sweetener if you need it. Add the sweetener to the hot cup, not the ice cup, so it dissolves best.
This thing is seriously a life saver for me.
If you're on the go a lot and you have a favourite place to get tea (or decaf coffee--it works for coffee too) order a small hot tea (or black hot coffee) and a cup of ice on the side and you can do the exact same thing to get an instant iced drink, usually cheaper than buying an iced drink! If you work somewhere that has a work fridge, you can also ask your coworkers if they'd be cool with you keeping a covered ice tray in the freezer so you always have ice to make tea or iced coffee with.