I’ve used Lipton gallon sized tea bags (https://www.amazon.com/Lipton-Gallon-Sized-Black-Iced-Unsweetened/dp/B00286KM2A, can find at some grocery stores) and fancy loose leaf from Adagio (https://www.adagio.com/black/assam_melody.html).

I like loose leaf but not the high price, so going to try this not fancy tea next: https://www.sfherb.com/China-Black-Tea--1-Lb_p_61.html. Also saw a recommendation for https://essentialorganicingredients.com/collections/tea.