Tomorrow I am running a pre-written Adventurers League module, Outlaws of the Iron Route, and it describes two people: one as a Talashan and one as a huge Calashite. I have worked out the Calishite was from Calimshan, but I can not work out where the Talashan come from. Where do I find this information?
Your best bet is older materials.
I would suggest the Forgotten Realms Campaign Setting from 3rd Edition. It's over three hundred pages of densely packed, tightly typeset information. It covers regions and countries in broad strokes, with paragraphs on individual cities. It spans a much larger area than any more recent material, and does a good job of it.
Obviously, the history has been updated quite a bit since then - the Spellplague in 4E made a mess of the Realms, though the Second Sundering brought much of it back for 5E. Some of the geography is different - your Tashalan1 is from Chult, which used be an island but is now attached to the mainland (again). That said, the basics of each nation are still similar, the traditions and culture of each country haven't drifted drastically.
1There's no such thing as a "Talashan", as far as I have seen. Many of the early DDEX modules suffered from a marked lack of proofreading. I strongly believe this is one of them.