To answer your question, I first went to the Gorean Index here.
Then Land Areas and then Islands, which brought me to this page.
This told me to go to Book 6, Page 137, where it says:
My four commercial voyages had been among the exchange islands, or free islands, in Thassa, administered as free ports by members of the Merchants. There were several such islands. Three, which I encountered frequently in my voyages, were Teletus, and, south of it, Tabor, named for the drum, which it resembles, and, to the north, among the northern islands, Scagnar. Others were Farnacium, Hulneth and Asperiche. I did not go as far south as Anango or Ianda, or as far north as Hunjer or Skjern, west of Torvaldsland.
Raiders of Gor    Book 6    Page 137
And then, here are all the other quotes which mention Ianda:
I had projected voyages for them to Ianda and to Torvaldsland. Each would be escorted by a medium galley. They would bring me, I conjectured, much riches.
Raiders of Gor    Book 6    Page 142
"Are not Cos and Tyros to attack?" I demanded.
"Yes! Yes!" he cried. "Yes!"
"And," I said, "what of Ar, and Ko-ro-ba, and Treve, and Thentis, and Turia, and Tharna and Tor!"
"Yes, yes, yes!" he whimpered.
"And," I said, "Teletus, Tabor, Scagnar!"
"Yes, yes!" he cried.
"And," I said, "Farnacium, and Hulneth and Asperiche! And Anango and Ianda, and Hunjer and Skjern and Torvaldsland! And Lydius and Helmutsport, and Schendi and Bazi!"
"Yes," he cried. "All are going to attack."
Raiders of Gor    Book 6    Pages 152 - 153
By the end of the second month the flag of Bosk, carried by one ship or another, was known from Ianda to Torvaldsland, and from the delta of the Vosk to the throne rooms of Cos and Tyros.
Raiders of Gor    Book 6    Page 191
I saw a blond giant from Torvaldsland, with braided hair, in shaggy jacket; a merchant from Tyros, hurrying, perfumed and sleek; seamen from Cos, and Port Kar, mortal enemies, yet passing one another without thought in the streets of Lydius; a black woman, veiled in yellow, borne in a palanquin by eight black warriors, perhaps from as far south as Anango or Ianda; two hunters, perhaps from Ar, cowled in the heads of forest panthers; a wood cutter from one of the villages north of Lydius, his sticks bound on his back; a peasant, from south of the Laurius, with a basket of suls, an intent, preoccupied scribe, lean and clad in the scribe's blue, with a scroll, perhaps come north for high fees to tutor the sons of rich men; a brown-clad, hearty fellow from Laura, some two hundred pasangs upriver; a slaver, with the medallion of Ar over his robes; two blond slave girls, clad in brief white, bells on their left ankles, walking together and laughing, speaking in the accents of Thentis; I saw even a warrior of the Tuchuks, from the distant, treeless plains of the south, though I did not know him; it was not by the epicanthic fold that I recognized him; it was by the courage scars, high on his angular cheekbones.
Hunters of Gor    Book 8    Pages 41 - 42
I regarded the vast map on the floor of the chamber. I could see, high on the map, Ax Glacier, Torvaldsland, and Hunjer and Skjern, and Helmutsport, and, lower, Kassau and the great green forests, and the river Laurius, and Laura and Lydius, and, lower, the islands, prominent among them Cos and Tyros; I saw the delta of the Vosk, and Port Kar, and, inland, Ko-ro-ba, the Towers of the Morning, and Thentis, in the mountains of Thentis, famed for her tarn flocks; and, to the south, among many other cities, Tharna, of the vast silver mines; I saw the Voltai Range, and Glorious Ar, and the Cartius and far to the south, Turia; and near the shore of Thassa, the islands of Anango and Ianda, and on the coast, the free ports of Schendi and Bazi. There were, on this map, hundreds of cities, and promontories and peninsulas, and rivers and inland lakes and seas.
Tribesmen of Gor    Book 10    Page 7
Coming about Point Schendi, behind us, some two pasangs astern, was a round ship. She flew the colors of Asperiche. Far to starboard we saw two other ships, a medium-class round ship and a heavy galley, the latter with red masts, both of Ianda.
Explorers of Gor    Book 13    Page 110
She was skillful and, I suspected, from the use of the hands and beads, had been trained in Ianda, a merchant island north of Anango.
Explorers of Gor    Book 13    Page 133
"The work commenced, months ago," said Msaliti.
"Surely this could not have been unknown to you," I said.
"The work was done in the shipyards of Ianda," he said. "I had heard rumors of such a project but did not understand the nature of the ships or that this Ubarate was involved. But now the ships are already moving upstream on the Nyoka."
Explorers of Gor    Book 13    Page 226
Yet I was certain that it had been he who had passed us in stealth in the night. There had been three vessels, of the sort which had been prepared in Ianda and brought to Schendi, and then to Lake Ushindi by way of the Nyoka, part of the fleet which Bila Huruma was organizing to support the explorations of Shaba, navigating the Ua, into the far interior.
Explorers of Gor    Book 13    Page 250
