typetive likes barley tea from Japan, called mugicha. It has the same dark roasted flavors as Postum but might need a hit of molasses or honey to emulate Postum’s taste and mouthfeel. You can get barley tea (roasted whole barley, which you boil right in the pan with the water) from Korean or Japanese grocery stores.
For something more similar to the full Postum experience, boogiebaby recommends a British product called Horlicks. Look for it in Asian grocery stores or ethnic stores catering to populations from the British diaspora, such as Jamaica and India. Along a similar vein, Nestlé makes a product called Nestum, which might substitute for Postum.
Board Link: Postum Substitute