There is an Indian grocery just down from Star Visa on Wichayanon Road the road that runs along the side of the US Consulate. They have plenty of Indian tea choices both loose and ready mixed. If you are a masala chai fan go for the GeeBees brand which comes both sweetened and non. if you want to make your own they have most of the herbs there including green cardamom.
Another source for herbs is the Indian place inside Kad Luang on the ground floor and there selection far surpasses that of the grocery place and they likely also sell loose tea.