You can buy Thangkas(Tibetan-style scrolls usually painted with figures of the Buddha or the Avalokitesvara), Tibetan Incense, Tsa-tsa(clay-made figures of the Buddha enshrined by Tibetans to ask for the blessings of the Buddha), Masks, Tibetan Knives, Tibetan carpets, Tibetan paper, wooden bowls, embroideries and bone carvings, etc.
You can buy almost all the things in the Barkhor Street which is the busiest Tibetan culture street in Tibet.